diff --git a/CHANGELOG.md b/CHANGELOG.md
index a0a4313db7..5ea9f39594 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,18 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com)
 and this project adheres to [Semantic Versioning](https://semver.org).
 
+# TBD - 2.3.6
+
+### Deprecated
+
+- Worksheet::getHashCode is no longer needed.
+
+### Fixed
+
+- Change hash code for worksheet. Backport of [PR #4207](https://github.com/PHPOffice/PhpSpreadsheet/pull/4207)
+- Retitling cloned worksheets. Backport of [PR #4302](https://github.com/PHPOffice/PhpSpreadsheet/pull/4302)
+
+
 # 2024-12-26 - 2.3.5
 
 ### Deprecated
diff --git a/src/PhpSpreadsheet/Cell/Cell.php b/src/PhpSpreadsheet/Cell/Cell.php
index 26b34cd940..f10af3ab6c 100644
--- a/src/PhpSpreadsheet/Cell/Cell.php
+++ b/src/PhpSpreadsheet/Cell/Cell.php
@@ -318,7 +318,7 @@ public function setValueExplicit(mixed $value, string $dataType = DataType::TYPE
         self::updateIfCellIsTableHeader($this->getParent()?->getParent(), $this, $oldValue, $value);
         $worksheet = $this->getWorksheet();
         $spreadsheet = $worksheet->getParent();
-        if (isset($spreadsheet)) {
+        if (isset($spreadsheet) && $spreadsheet->getIndex($worksheet, true) >= 0) {
             $originalSelected = $worksheet->getSelectedCells();
             $activeSheetIndex = $spreadsheet->getActiveSheetIndex();
             $style = $this->getStyle();
diff --git a/src/PhpSpreadsheet/ReferenceHelper.php b/src/PhpSpreadsheet/ReferenceHelper.php
index 8b2f76cc29..45c131b6cc 100644
--- a/src/PhpSpreadsheet/ReferenceHelper.php
+++ b/src/PhpSpreadsheet/ReferenceHelper.php
@@ -921,7 +921,7 @@ private function updateNamedRange(DefinedName $definedName, Worksheet $worksheet
     {
         $cellAddress = $definedName->getValue();
         $asFormula = ($cellAddress[0] === '=');
-        if ($definedName->getWorksheet() !== null && $definedName->getWorksheet()->getHashCode() === $worksheet->getHashCode()) {
+        if ($definedName->getWorksheet() !== null && $definedName->getWorksheet()->getHashInt() === $worksheet->getHashInt()) {
             /**
              * If we delete the entire range that is referenced by a Named Range, MS Excel sets the value to #REF!
              * PhpSpreadsheet still only does a basic adjustment, so the Named Range will still reference Cells.
@@ -940,7 +940,7 @@ private function updateNamedRange(DefinedName $definedName, Worksheet $worksheet
 
     private function updateNamedFormula(DefinedName $definedName, Worksheet $worksheet, string $beforeCellAddress, int $numberOfColumns, int $numberOfRows): void
     {
-        if ($definedName->getWorksheet() !== null && $definedName->getWorksheet()->getHashCode() === $worksheet->getHashCode()) {
+        if ($definedName->getWorksheet() !== null && $definedName->getWorksheet()->getHashInt() === $worksheet->getHashInt()) {
             /**
              * If we delete the entire range that is referenced by a Named Formula, MS Excel sets the value to #REF!
              * PhpSpreadsheet still only does a basic adjustment, so the Named Formula will still reference Cells.
diff --git a/src/PhpSpreadsheet/Spreadsheet.php b/src/PhpSpreadsheet/Spreadsheet.php
index bcea8e6a74..dc228d283c 100644
--- a/src/PhpSpreadsheet/Spreadsheet.php
+++ b/src/PhpSpreadsheet/Spreadsheet.php
@@ -511,7 +511,7 @@ public function getActiveSheet(): Worksheet
     public function createSheet(?int $sheetIndex = null): Worksheet
     {
         $newSheet = new Worksheet($this);
-        $this->addSheet($newSheet, $sheetIndex);
+        $this->addSheet($newSheet, $sheetIndex, true);
 
         return $newSheet;
     }
@@ -532,8 +532,20 @@ public function sheetNameExists(string $worksheetName): bool
      * @param Worksheet $worksheet The worksheet to add
      * @param null|int $sheetIndex Index where sheet should go (0,1,..., or null for last)
      */
-    public function addSheet(Worksheet $worksheet, ?int $sheetIndex = null): Worksheet
+    public function addSheet(Worksheet $worksheet, ?int $sheetIndex = null, bool $retitleIfNeeded = false): Worksheet
     {
+        if ($retitleIfNeeded) {
+            $title = $worksheet->getTitle();
+            if ($this->sheetNameExists($title)) {
+                $i = 1;
+                $newTitle = "$title $i";
+                while ($this->sheetNameExists($newTitle)) {
+                    ++$i;
+                    $newTitle = "$title $i";
+                }
+                $worksheet->setTitle($newTitle);
+            }
+        }
         if ($this->sheetNameExists($worksheet->getTitle())) {
             throw new Exception(
                 "Workbook already contains a worksheet named '{$worksheet->getTitle()}'. Rename this worksheet first."
@@ -657,13 +669,17 @@ public function getSheetByNameOrThrow(string $worksheetName): Worksheet
      *
      * @return int index
      */
-    public function getIndex(Worksheet $worksheet): int
+    public function getIndex(Worksheet $worksheet, bool $noThrow = false): int
     {
+        $wsHash = $worksheet->getHashInt();
         foreach ($this->workSheetCollection as $key => $value) {
-            if ($value->getHashCode() === $worksheet->getHashCode()) {
+            if ($value->getHashInt() === $wsHash) {
                 return $key;
             }
         }
+        if ($noThrow) {
+            return -1;
+        }
 
         throw new Exception('Sheet does not exist.');
     }
diff --git a/src/PhpSpreadsheet/Worksheet/BaseDrawing.php b/src/PhpSpreadsheet/Worksheet/BaseDrawing.php
index 0cd4448672..81bcf73c83 100644
--- a/src/PhpSpreadsheet/Worksheet/BaseDrawing.php
+++ b/src/PhpSpreadsheet/Worksheet/BaseDrawing.php
@@ -416,7 +416,7 @@ public function getHashCode(): string
         return md5(
             $this->name
             . $this->description
-            . (($this->worksheet === null) ? '' : $this->worksheet->getHashCode())
+            . (($this->worksheet === null) ? '' : (string) $this->worksheet->getHashInt())
             . $this->coordinates
             . $this->offsetX
             . $this->offsetY
diff --git a/src/PhpSpreadsheet/Worksheet/Worksheet.php b/src/PhpSpreadsheet/Worksheet/Worksheet.php
index f8e9415435..a602fa8097 100644
--- a/src/PhpSpreadsheet/Worksheet/Worksheet.php
+++ b/src/PhpSpreadsheet/Worksheet/Worksheet.php
@@ -20,7 +20,6 @@
 use PhpOffice\PhpSpreadsheet\Comment;
 use PhpOffice\PhpSpreadsheet\DefinedName;
 use PhpOffice\PhpSpreadsheet\Exception;
-use PhpOffice\PhpSpreadsheet\IComparable;
 use PhpOffice\PhpSpreadsheet\ReferenceHelper;
 use PhpOffice\PhpSpreadsheet\RichText\RichText;
 use PhpOffice\PhpSpreadsheet\Shared;
@@ -32,7 +31,7 @@
 use PhpOffice\PhpSpreadsheet\Style\Protection as StyleProtection;
 use PhpOffice\PhpSpreadsheet\Style\Style;
 
-class Worksheet implements IComparable
+class Worksheet
 {
     // Break types
     public const BREAK_NONE = 0;
@@ -305,15 +304,10 @@ class Worksheet implements IComparable
      */
     private ?Color $tabColor = null;
 
-    /**
-     * Dirty flag.
-     */
-    private bool $dirty = true;
-
     /**
      * Hash.
      */
-    private string $hash;
+    private int $hash;
 
     /**
      * CodeName.
@@ -327,6 +321,7 @@ public function __construct(?Spreadsheet $parent = null, string $title = 'Worksh
     {
         // Set parent and title
         $this->parent = $parent;
+        $this->hash = spl_object_id($this);
         $this->setTitle($title, false);
         // setTitle can change $pTitle
         $this->setCodeName($this->getTitle());
@@ -383,6 +378,12 @@ public function __destruct()
         unset($this->rowDimensions, $this->columnDimensions, $this->tableCollection, $this->drawingCollection, $this->chartCollection, $this->autoFilter);
     }
 
+    public function __wakeup(): void
+    {
+        $this->hash = spl_object_id($this);
+        $this->parent = null;
+    }
+
     /**
      * Return the cell collection.
      */
@@ -862,7 +863,7 @@ public function setTitle(string $title, bool $updateFormulaCellReferences = true
             // Syntax check
             self::checkSheetTitle($title);
 
-            if ($this->parent) {
+            if ($this->parent && $this->parent->getIndex($this, true) >= 0) {
                 // Is there already such sheet name?
                 if ($this->parent->sheetNameExists($title)) {
                     // Use name, but append with lowest possible integer
@@ -891,9 +892,8 @@ public function setTitle(string $title, bool $updateFormulaCellReferences = true
 
         // Set title
         $this->title = $title;
-        $this->dirty = true;
 
-        if ($this->parent && $this->parent->getCalculationEngine()) {
+        if ($this->parent && $this->parent->getIndex($this, true) >= 0 && $this->parent->getCalculationEngine()) {
             // New title
             $newTitle = $this->getTitle();
             $this->parent->getCalculationEngine()
@@ -1026,7 +1026,6 @@ public function getProtection(): Protection
     public function setProtection(Protection $protection): static
     {
         $this->protection = $protection;
-        $this->dirty = true;
 
         return $this;
     }
@@ -2994,7 +2993,7 @@ private function validateNamedRange(string $definedName, bool $returnNullIfInval
 
         if ($namedRange->getLocalOnly()) {
             $worksheet = $namedRange->getWorksheet();
-            if ($worksheet === null || $this->getHashCode() !== $worksheet->getHashCode()) {
+            if ($worksheet === null || $this->hash !== $worksheet->getHashInt()) {
                 if ($returnNullIfInvalid) {
                     return null;
                 }
@@ -3131,17 +3130,15 @@ public function garbageCollect(): static
     }
 
     /**
-     * Get hash code.
-     *
-     * @return string Hash code
+     * @deprecated 3.5.0 use getHashInt instead.
      */
     public function getHashCode(): string
     {
-        if ($this->dirty) {
-            $this->hash = md5($this->title . $this->autoFilter . ($this->protection->isProtectionEnabled() ? 't' : 'f') . __CLASS__);
-            $this->dirty = false;
-        }
+        return (string) $this->hash;
+    }
 
+    public function getHashInt(): int
+    {
         return $this->hash;
     }
 
@@ -3470,6 +3467,7 @@ public function __clone()
                 }
             }
         }
+        $this->hash = spl_object_id($this);
     }
 
     /**
diff --git a/src/PhpSpreadsheet/Writer/Xlsx/Style.php b/src/PhpSpreadsheet/Writer/Xlsx/Style.php
index daac5d020c..a31b8e2498 100644
--- a/src/PhpSpreadsheet/Writer/Xlsx/Style.php
+++ b/src/PhpSpreadsheet/Writer/Xlsx/Style.php
@@ -142,6 +142,7 @@ public function writeStyles(Spreadsheet $spreadsheet): string
 
         // dxf
         for ($i = 0; $i < $this->getParentWriter()->getStylesConditionalHashTable()->count(); ++$i) {
+            /** @var ?Conditional */
             $thisstyle = $this->getParentWriter()->getStylesConditionalHashTable()->getByIndex($i);
             if ($thisstyle !== null) {
                 $this->writeCellStyleDxf($objWriter, $thisstyle->getStyle());
diff --git a/tests/PhpSpreadsheetTests/Shared/DateTest.php b/tests/PhpSpreadsheetTests/Shared/DateTest.php
index 8da86e98ee..d747785917 100644
--- a/tests/PhpSpreadsheetTests/Shared/DateTest.php
+++ b/tests/PhpSpreadsheetTests/Shared/DateTest.php
@@ -8,6 +8,7 @@
 use DateTimeZone;
 use PhpOffice\PhpSpreadsheet\Exception;
 use PhpOffice\PhpSpreadsheet\Shared\Date;
+use PhpOffice\PhpSpreadsheet\Spreadsheet;
 use PhpOffice\PhpSpreadsheet\Style\NumberFormat;
 use PHPUnit\Framework\TestCase;
 
@@ -206,7 +207,7 @@ public function testVarious(): void
         $date = Date::PHPToExcel('2020-01-01');
         self::assertEquals(43831.0, $date);
 
-        $spreadsheet = new \PhpOffice\PhpSpreadsheet\Spreadsheet();
+        $spreadsheet = new Spreadsheet();
         $sheet = $spreadsheet->getActiveSheet();
         $sheet->setCellValue('B1', 'x');
         /** @var float|int|string */
@@ -248,5 +249,6 @@ public function testVarious(): void
             ->getNumberFormat()
             ->setFormatCode('yyyy-mm-dd');
         self::assertFalse(Date::isDateTime($cella4));
+        $spreadsheet->disconnectWorksheets();
     }
 }
diff --git a/tests/PhpSpreadsheetTests/Shared/StringHelperInvalidCharTest.php b/tests/PhpSpreadsheetTests/Shared/StringHelperInvalidCharTest.php
index eb46beaf39..13a58dfe19 100644
--- a/tests/PhpSpreadsheetTests/Shared/StringHelperInvalidCharTest.php
+++ b/tests/PhpSpreadsheetTests/Shared/StringHelperInvalidCharTest.php
@@ -43,5 +43,6 @@ public function testInvalidChar(): void
                 $sheet->getCell("A$row")->getValue()
             );
         }
+        $spreadsheet->disconnectWorksheets();
     }
 }
diff --git a/tests/PhpSpreadsheetTests/Style/BorderRangeTest.php b/tests/PhpSpreadsheetTests/Style/BorderRangeTest.php
index 844fa3fb50..8c50ddc17d 100644
--- a/tests/PhpSpreadsheetTests/Style/BorderRangeTest.php
+++ b/tests/PhpSpreadsheetTests/Style/BorderRangeTest.php
@@ -62,6 +62,7 @@ public function testBorderRangeInAction(): void
                 }
             }
         }
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testBorderRangeDirectly(): void
@@ -71,5 +72,6 @@ public function testBorderRangeDirectly(): void
         $sheet = $spreadsheet->getActiveSheet();
         $style = $sheet->getStyle('A1:C1')->getBorders()->getTop()->setBorderStyle(Border::BORDER_THIN);
         self::assertSame('A1:C1', $style->getSelectedCells(), 'getSelectedCells should not change after a style operation on a border range');
+        $spreadsheet->disconnectWorksheets();
     }
 }
diff --git a/tests/PhpSpreadsheetTests/Style/BorderTest.php b/tests/PhpSpreadsheetTests/Style/BorderTest.php
index 9f18f07668..e6684b1a93 100644
--- a/tests/PhpSpreadsheetTests/Style/BorderTest.php
+++ b/tests/PhpSpreadsheetTests/Style/BorderTest.php
@@ -31,6 +31,7 @@ public function testAllBorders(): void
         self::assertSame(Border::BORDER_THIN, $borders->getRight()->getBorderStyle());
         self::assertSame(Border::BORDER_THIN, $borders->getLeft()->getBorderStyle());
         self::assertSame(Border::BORDER_NONE, $borders->getDiagonal()->getBorderStyle());
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testAllBordersArray(): void
@@ -45,6 +46,7 @@ public function testAllBordersArray(): void
         self::assertSame(Border::BORDER_THIN, $borders->getRight()->getBorderStyle());
         self::assertSame(Border::BORDER_THIN, $borders->getLeft()->getBorderStyle());
         self::assertSame(Border::BORDER_NONE, $borders->getDiagonal()->getBorderStyle());
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testAllBordersArrayNotSupervisor(): void
@@ -86,6 +88,7 @@ public function testOutline(): void
         self::assertSame(Border::BORDER_THIN, $sheet->getCell('B2')->getStyle()->getBorders()->getBottom()->getBorderStyle());
         self::assertSame(Border::BORDER_NONE, $sheet->getCell('B2')->getStyle()->getBorders()->getLeft()->getBorderStyle());
         self::assertSame(Border::BORDER_NONE, $sheet->getCell('B2')->getStyle()->getBorders()->getTop()->getBorderStyle());
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testInside(): void
@@ -115,6 +118,7 @@ public function testInside(): void
         self::assertSame(Border::BORDER_NONE, $sheet->getCell('B2')->getStyle()->getBorders()->getBottom()->getBorderStyle());
         self::assertSame(Border::BORDER_THIN, $sheet->getCell('B2')->getStyle()->getBorders()->getLeft()->getBorderStyle());
         self::assertSame(Border::BORDER_THIN, $sheet->getCell('B2')->getStyle()->getBorders()->getTop()->getBorderStyle());
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testHorizontal(): void
@@ -144,6 +148,7 @@ public function testHorizontal(): void
         self::assertSame(Border::BORDER_NONE, $sheet->getCell('B2')->getStyle()->getBorders()->getBottom()->getBorderStyle());
         self::assertSame(Border::BORDER_NONE, $sheet->getCell('B2')->getStyle()->getBorders()->getLeft()->getBorderStyle());
         self::assertSame(Border::BORDER_THIN, $sheet->getCell('B2')->getStyle()->getBorders()->getTop()->getBorderStyle());
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testVertical(): void
@@ -173,6 +178,7 @@ public function testVertical(): void
         self::assertSame(Border::BORDER_NONE, $sheet->getCell('B2')->getStyle()->getBorders()->getBottom()->getBorderStyle());
         self::assertSame(Border::BORDER_THIN, $sheet->getCell('B2')->getStyle()->getBorders()->getLeft()->getBorderStyle());
         self::assertSame(Border::BORDER_NONE, $sheet->getCell('B2')->getStyle()->getBorders()->getTop()->getBorderStyle());
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testNoSupervisorAllBorders(): void
@@ -233,6 +239,7 @@ public function testBorderStyle(): void
         $border->setBorderStyle(Border::BORDER_THIN)->setColor(new Color('FFFF0000'));
         self::assertEquals('FFFF0000', $border->getColor()->getARGB());
         self::assertEquals(Border::BORDER_THIN, $border->getBorderStyle());
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testDiagonalDirection(): void
@@ -245,5 +252,6 @@ public function testDiagonalDirection(): void
 
         self::assertSame(Border::BORDER_MEDIUM, $borders->getDiagonal()->getBorderStyle());
         self::assertSame(Borders::DIAGONAL_BOTH, $borders->getDiagonalDirection());
+        $spreadsheet->disconnectWorksheets();
     }
 }
diff --git a/tests/PhpSpreadsheetTests/Style/ConditionalFormatting/Wizard/WizardFactoryTest.php b/tests/PhpSpreadsheetTests/Style/ConditionalFormatting/Wizard/WizardFactoryTest.php
index 04681b39a0..5002bcaa1a 100644
--- a/tests/PhpSpreadsheetTests/Style/ConditionalFormatting/Wizard/WizardFactoryTest.php
+++ b/tests/PhpSpreadsheetTests/Style/ConditionalFormatting/Wizard/WizardFactoryTest.php
@@ -66,6 +66,7 @@ public function testWizardFromConditional(string $sheetName, string $cellAddress
             $wizard = Wizard::fromConditional($conditional);
             self::assertEquals($expectedWizads[$index], $wizard::class);
         }
+        $spreadsheet->disconnectWorksheets();
     }
 
     public static function conditionalProvider(): array
diff --git a/tests/PhpSpreadsheetTests/Style/StyleTest.php b/tests/PhpSpreadsheetTests/Style/StyleTest.php
index be67a5e792..6446bcf991 100644
--- a/tests/PhpSpreadsheetTests/Style/StyleTest.php
+++ b/tests/PhpSpreadsheetTests/Style/StyleTest.php
@@ -24,6 +24,7 @@ public function testStyleOddMethods(): void
         $styleArray = ['alignment' => ['textRotation' => 45]];
         $outArray = $cell1style->getStyleArray($styleArray);
         self::assertEquals($styleArray, $outArray['quotePrefix']);
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testStyleColumn(): void
@@ -58,6 +59,7 @@ public function testStyleColumn(): void
         self::assertTrue($sheet->getStyle('A1')->getFont()->getItalic());
         self::assertTrue($sheet->getStyle('B2')->getFont()->getItalic());
         self::assertFalse($sheet->getStyle('C3')->getFont()->getItalic());
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testStyleIsReused(): void
@@ -81,6 +83,7 @@ public function testStyleIsReused(): void
         $spreadsheet->garbageCollect();
 
         self::assertCount(3, $spreadsheet->getCellXfCollection());
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testStyleRow(): void
@@ -115,6 +118,7 @@ public function testStyleRow(): void
         self::assertFalse($sheet->getStyle('A1')->getFont()->getItalic());
         self::assertTrue($sheet->getStyle('B2')->getFont()->getItalic());
         self::assertTrue($sheet->getStyle('C3')->getFont()->getItalic());
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testIssue1712A(): void
@@ -137,6 +141,7 @@ public function testIssue1712A(): void
             ->setRGB($rgb);
         self::assertEquals($rgb, $sheet->getCell('A1')->getStyle()->getFill()->getStartColor()->getRGB());
         self::assertEquals($rgb, $sheet->getCell('B1')->getStyle()->getFill()->getStartColor()->getRGB());
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testIssue1712B(): void
@@ -159,6 +164,7 @@ public function testIssue1712B(): void
         $sheet->fromArray(['OK', 'KO']);
         self::assertEquals($rgb, $sheet->getCell('A1')->getStyle()->getFill()->getStartColor()->getRGB());
         self::assertEquals($rgb, $sheet->getCell('B1')->getStyle()->getFill()->getStartColor()->getRGB());
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testStyleLoopUpwards(): void
@@ -184,6 +190,7 @@ public function testStyleLoopUpwards(): void
         self::assertFalse($sheet->getStyle('A1')->getFont()->getBold());
         self::assertFalse($sheet->getStyle('B2')->getFont()->getBold());
         self::assertTrue($sheet->getStyle('C3')->getFont()->getBold());
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testStyleCellAddressObject(): void
@@ -195,6 +202,7 @@ public function testStyleCellAddressObject(): void
         $style->getNumberFormat()->setFormatCode(NumberFormat::FORMAT_DATE_YYYYMMDDSLASH);
 
         self::assertSame(NumberFormat::FORMAT_DATE_YYYYMMDDSLASH, $style->getNumberFormat()->getFormatCode());
+        $spreadsheet->disconnectWorksheets();
     }
 
     public function testStyleCellRangeObject(): void
@@ -208,5 +216,6 @@ public function testStyleCellRangeObject(): void
         $style->getNumberFormat()->setFormatCode(NumberFormat::FORMAT_DATE_YYYYMMDDSLASH);
 
         self::assertSame(NumberFormat::FORMAT_DATE_YYYYMMDDSLASH, $style->getNumberFormat()->getFormatCode());
+        $spreadsheet->disconnectWorksheets();
     }
 }
diff --git a/tests/PhpSpreadsheetTests/Worksheet/AutoSizeTest.php b/tests/PhpSpreadsheetTests/Worksheet/AutoSizeTest.php
index 4a4f488bb3..558590bfb3 100644
--- a/tests/PhpSpreadsheetTests/Worksheet/AutoSizeTest.php
+++ b/tests/PhpSpreadsheetTests/Worksheet/AutoSizeTest.php
@@ -13,16 +13,14 @@
 
 class AutoSizeTest extends TestCase
 {
-    protected Spreadsheet $spreadsheet;
+    private Spreadsheet $spreadsheet;
 
-    protected Worksheet $worksheet;
+    private Worksheet $worksheet;
 
     protected function setUp(): void
     {
-        parent::setUp();
-
-        $spreadsheet = new Spreadsheet();
-        $this->worksheet = $spreadsheet->getActiveSheet();
+        $this->spreadsheet = new Spreadsheet();
+        $this->worksheet = $this->spreadsheet->getActiveSheet();
 
         $this->worksheet->setCellValue('A1', 'YEAR')
             ->setCellValue('B1', 'QUARTER')
@@ -44,7 +42,13 @@ protected function setUp(): void
         }
     }
 
-    protected function setTable(): Table
+    protected function tearDown(): void
+    {
+        $this->spreadsheet->disconnectWorksheets();
+        unset($this->spreadsheet, $this->worksheet);
+    }
+
+    private function setTable(): Table
     {
         $table = new Table('A1:D5', 'Sales_Data');
         $tableStyle = new TableStyle();
@@ -55,7 +59,7 @@ protected function setTable(): Table
         return $table;
     }
 
-    protected function readColumnSizes(): array
+    private function readColumnSizes(): array
     {
         $columnSizes = [];
         $toColumn = $this->worksheet->getHighestColumn();
diff --git a/tests/PhpSpreadsheetTests/Worksheet/CloneTest.php b/tests/PhpSpreadsheetTests/Worksheet/CloneTest.php
new file mode 100644
index 0000000000..221ec2fb3b
--- /dev/null
+++ b/tests/PhpSpreadsheetTests/Worksheet/CloneTest.php
@@ -0,0 +1,75 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpOffice\PhpSpreadsheetTests\Worksheet;
+
+use PhpOffice\PhpSpreadsheet\Exception as SpreadsheetException;
+use PhpOffice\PhpSpreadsheet\Spreadsheet;
+use PhpOffice\PhpSpreadsheet\Worksheet\Worksheet;
+use PHPUnit\Framework\TestCase;
+
+class CloneTest extends TestCase
+{
+    private Spreadsheet $spreadsheet;
+
+    protected function setUp(): void
+    {
+        $this->spreadsheet = new Spreadsheet();
+    }
+
+    protected function tearDown(): void
+    {
+        $this->spreadsheet->disconnectWorksheets();
+        unset($this->spreadsheet);
+    }
+
+    public function testUnattachedIndex(): void
+    {
+        $sheet1 = $this->spreadsheet->getActiveSheet();
+        $sheet1->getCell('A1')->setValue(10);
+        $sheet2 = clone $sheet1;
+        $sheet2->getCell('A1')->setValue(20);
+        self::assertSame(0, $this->spreadsheet->getIndex($sheet1));
+        $idx = $this->spreadsheet->getIndex($sheet2, true);
+        self::assertSame(-1, $idx);
+        $sheet2->setTitle('clone');
+        $this->spreadsheet->addSheet($sheet2);
+        $idx = $this->spreadsheet->getIndex($sheet2, true);
+        self::assertSame(1, $idx);
+        self::assertSame(10, $this->spreadsheet->getSheet(0)->getCell('A1')->getValue());
+        self::assertSame(20, $this->spreadsheet->getSheet(1)->getCell('A1')->getValue());
+    }
+
+    public function testGetCloneIndex(): void
+    {
+        $this->expectException(SpreadsheetException::class);
+        $this->expectExceptionMessage('Sheet does not exist');
+        $sheet1 = $this->spreadsheet->getActiveSheet();
+        $sheet1->getCell('A1')->setValue(10);
+        $sheet2 = clone $sheet1;
+        $this->spreadsheet
+            ->getSheet($this->spreadsheet->getIndex($sheet2))
+            ->setCellValue('A1', 100);
+    }
+
+    public function testSerialize1(): void
+    {
+        // If worksheet attached to spreadsheet, can't serialize it.
+        $this->expectException(SpreadsheetException::class);
+        $this->expectExceptionMessage('cannot be serialized');
+        $sheet1 = $this->spreadsheet->getActiveSheet();
+        serialize($sheet1);
+    }
+
+    public function testSerialize2(): void
+    {
+        $sheet1 = new Worksheet();
+        $sheet1->getCell('A1')->setValue(10);
+        $serialized = serialize($sheet1);
+        /** @var Worksheet */
+        $newSheet = unserialize($serialized);
+        self::assertSame(10, $newSheet->getCell('A1')->getValue());
+        self::assertNotEquals($newSheet->getHashInt(), $sheet1->getHashInt());
+    }
+}
diff --git a/tests/PhpSpreadsheetTests/Worksheet/Issue641Test.php b/tests/PhpSpreadsheetTests/Worksheet/Issue641Test.php
new file mode 100644
index 0000000000..bbb6c8bf00
--- /dev/null
+++ b/tests/PhpSpreadsheetTests/Worksheet/Issue641Test.php
@@ -0,0 +1,82 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpOffice\PhpSpreadsheetTests\Worksheet;
+
+use PhpOffice\PhpSpreadsheet\Spreadsheet;
+use PHPUnit\Framework\TestCase;
+
+class Issue641Test extends TestCase
+{
+    /**
+     * Problem cloning sheet referred to in formulas.
+     */
+    public function testIssue641(): void
+    {
+        $xlsx = new Spreadsheet();
+        $xlsx->removeSheetByIndex(0);
+        $availableWs = [];
+
+        $worksheet = $xlsx->createSheet();
+        $worksheet->setTitle('Condensed A');
+        $worksheet->getCell('A1')->setValue("=SUM('Detailed A'!A1:A10)");
+        $worksheet->getCell('A2')->setValue(mt_rand(1, 30));
+        $availableWs[] = 'Condensed A';
+
+        $worksheet = $xlsx->createSheet();
+        $worksheet->setTitle('Condensed B');
+        $worksheet->getCell('A1')->setValue("=SUM('Detailed B'!A1:A10)");
+        $worksheet->getCell('A2')->setValue(mt_rand(1, 30));
+        $availableWs[] = 'Condensed B';
+
+        // at this point the value in worksheet 'Condensed B' cell A1 is
+        // =SUM('Detailed B'!A1:A10)
+
+        // worksheet in question is cloned and totals are attached
+        $totalWs1 = clone $xlsx->getSheet($xlsx->getSheetCount() - 1);
+        $totalWs1->setTitle('Condensed Total');
+        $xlsx->addSheet($totalWs1);
+        $formula = '=';
+        foreach ($availableWs as $ws) {
+            $formula .= sprintf("+'%s'!A2", $ws);
+        }
+        $totalWs1->getCell('A1')->setValue("=SUM('Detailed Total'!A1:A10)");
+        $totalWs1->getCell('A2')->setValue($formula);
+
+        $availableWs = [];
+
+        $worksheet = $xlsx->createSheet();
+        $worksheet->setTitle('Detailed A');
+        for ($step = 1; $step <= 10; ++$step) {
+            $worksheet->getCell("A{$step}")->setValue(mt_rand(1, 30));
+        }
+        $availableWs[] = 'Detailed A';
+
+        $worksheet = $xlsx->createSheet();
+        $worksheet->setTitle('Detailed B');
+        for ($step = 1; $step <= 10; ++$step) {
+            $worksheet->getCell("A{$step}")->setValue(mt_rand(1, 30));
+        }
+        $availableWs[] = 'Detailed B';
+
+        $totalWs2 = clone $xlsx->getSheet($xlsx->getSheetCount() - 1);
+        $totalWs2->setTitle('Detailed Total');
+        $xlsx->addSheet($totalWs2);
+
+        for ($step = 1; $step <= 10; ++$step) {
+            $formula = '=';
+            foreach ($availableWs as $ws) {
+                $formula .= sprintf("+'%s'!A%s", $ws, $step);
+            }
+            $totalWs2->getCell("A{$step}")->setValue($formula);
+        }
+
+        self::assertSame("=SUM('Detailed A'!A1:A10)", $xlsx->getSheetByName('Condensed A')?->getCell('A1')?->getValue());
+        self::assertSame("=SUM('Detailed B'!A1:A10)", $xlsx->getSheetByName('Condensed B')?->getCell('A1')?->getValue());
+        self::assertSame("=SUM('Detailed Total'!A1:A10)", $xlsx->getSheetByName('Condensed Total')?->getCell('A1')?->getValue());
+        self::assertSame("=+'Detailed A'!A1+'Detailed B'!A1", $xlsx->getSheetByName('Detailed Total')?->getCell('A1')?->getValue());
+
+        $xlsx->disconnectWorksheets();
+    }
+}