Closes #6621: Fix IfcRelAssignsToProduct on duplicate

When duplicating a product+annotation pair linked via
IfcRelAssignsToProduct, copy_class's generic else-branch
appended the new annotation to every existing rel that
referenced the original, leaving both old and new products
sharing stale RelatedObjects.

Adds fix_assigns_to_product_after_duplication to
tool.Geometry, called at the end of duplicate_ifc_objects.
It corrects the three cases: new annotation in old product's
rel (remove it), stale original in new product's rel (remove
it), and annotation-only duplicate (build a fresh unique rel).

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/tool/geometry.py

@@ -2533,6 +2533,7 @@ class Geometry(bonsai.core.tool.Geometry):
         # is complete.
         cls._recalculate_walls_with_new_connections(old_to_new)
 
+        cls.fix_assigns_to_product_after_duplication(old_to_new)
         bonsai.bim.handler.refresh_ui_data()
         tool.Root.reload_grid_decorator()
         return old_to_new, new_active_obj or active_object
@@ -2825,6 +2826,49 @@ class Geometry(bonsai.core.tool.Geometry):
                 if entity in old_to_new.keys() or single_obj:
                     cls.remove_connection(connection)
 
+    @classmethod
+    def fix_assigns_to_product_after_duplication(
+        cls, old_to_new: dict[ifcopenshell.entity_instance, list[ifcopenshell.entity_instance]]
+    ) -> None:
+        """Fix IfcRelAssignsToProduct relationships broken by copy_class duplication.
+
+        copy_class hits the generic else-branch for IfcRelAssignsToProduct and
+        appends the new element into every existing rel that referenced the original,
+        causing both old and new products to share stale RelatedObjects lists.
+
+        Three cases, identified by whether the RelatingProduct was also duplicated:
+          A) RelatingProduct is OLD → new annotation was incorrectly appended → remove it.
+          B) RelatingProduct is NEW → stale original annotation was carried over → remove it.
+          C) RelatingProduct unchanged → create a genuinely new rel for the new annotation.
+        """
+        ifc_file = tool.Ifc.get()
+        old_elements = set(old_to_new.keys())
+        new_to_old = {new: old for old, new_list in old_to_new.items() for new in new_list}
+        new_elements = set(new_to_old.keys())
+
+        for new_elem in new_elements:
+            original_elem = new_to_old[new_elem]
+            for rel in list(ifc_file.get_inverse(new_elem)):
+                if not rel.is_a("IfcRelAssignsToProduct") or new_elem not in rel.RelatedObjects:
+                    continue
+                relating_product = rel.RelatingProduct
+                if relating_product in old_elements:
+                    # Case A: remove new elem from the old product's rel
+                    rel.RelatedObjects = [e for e in rel.RelatedObjects if e != new_elem]
+                elif relating_product in new_elements:
+                    # Case B: remove the stale original from the new product's rel
+                    rel.RelatedObjects = [e for e in rel.RelatedObjects if e != original_elem]
+                else:
+                    # Case C: product not duplicated — build a fresh unique rel
+                    rel.RelatedObjects = [e for e in rel.RelatedObjects if e != new_elem]
+                    ifc_file.create_entity(
+                        "IfcRelAssignsToProduct",
+                        GlobalId=ifcopenshell.guid.new(),
+                        OwnerHistory=rel.OwnerHistory,
+                        RelatedObjects=[new_elem],
+                        RelatingProduct=relating_product,
+                    )
+
     @classmethod
     def remove_linked_aggregate_data(cls, old_to_new):
         ifc_file = tool.Ifc.get()