Update src/highdicom/seg/sop.py

Co-authored-by: Chris Bridge <[email protected]>

src/highdicom/seg/sop.py

@@ -2174,7 +2174,7 @@ def _get_ref_instance_uids(self) -> List[Tuple[str, str, str]]:
                 f'duplicated referenced SOP Instance UID items: {display_str}.'
             )
 
-        return unique_instance_data.copy()
+        return unique_instance_data
 
     def _build_luts(self) -> None:
         """Build lookup tables for efficient querying.