checker: check duplicate interfaces when implements for struct

vlib/v/checker/struct.v

@@ -333,6 +333,7 @@ fn (mut c Checker) struct_decl(mut node ast.StructDecl) {
 		// XTODO2
 		// cgen error if I use `println(sym)` without handling the option with `or{}`
 		struct_type := c.table.find_type_idx(node.name) // or { panic(err) }
+		mut names_used := []string{}
 		for t in node.implements_types {
 			t_sym := c.table.sym(t.typ)
 			if t_sym.info is ast.Interface {
@@ -350,6 +351,12 @@ fn (mut c Checker) struct_decl(mut node ast.StructDecl) {
 						}
 					}
 				}
+				variant_name := c.table.type_to_str(t.typ)
+				if variant_name in names_used {
+					c.error('struct type ${node.name} cannot implement interface `${t_sym.name} more than once`',
+						t.pos)
+				}
+				names_used << variant_name
 			} else {
 				c.error('`${t_sym.name}` is not an interface type', t.pos)
 			}

vlib/v/checker/tests/struct_implements_interface_more_than_once_err.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/struct_implements_interface_more_than_once_err.vv:3:31: error: struct type Abc cannot implement interface `io.Writer more than once`
+    1 | import io { Writer }
+    2 | 
+    3 | struct Abc implements Writer, Writer, Writer {
+      |                               ~~~~~~
+    4 |     aaa string
+    5 | }
+vlib/v/checker/tests/struct_implements_interface_more_than_once_err.vv:12:31: error: struct type Def cannot implement interface `io.Writer more than once`
+   10 | 
+   11 | // vfmt off
+   12 | struct Def implements Writer, io.Writer {
+      |                               ~~
+   13 |     aaa string
+   14 | }

vlib/v/checker/tests/struct_implements_interface_more_than_once_err.vv

@@ -0,0 +1,19 @@
+import io { Writer }
+
+struct Abc implements Writer, Writer, Writer {
+	aaa string
+}
+
+fn (a Abc) write(buf []u8) !int {
+	return 42
+}
+
+// vfmt off
+struct Def implements Writer, io.Writer {
+	aaa string
+}
+// vfmt on
+
+fn (a Def) write(buf []u8) !int {
+	return 42
+}