Fix #5271. Prevent duplicate entries when getting a decomposition.

src/ifcopenshell-python/ifcopenshell/util/element.py

@@ -947,7 +947,7 @@ def get_structure_referenced_elements(structure: ifcopenshell.entity_instance) -
     return referenced
 
 
-def get_decomposition(element: ifcopenshell.entity_instance, is_recursive=True) -> list[ifcopenshell.entity_instance]:
+def get_decomposition(element: ifcopenshell.entity_instance, is_recursive=True) -> set[ifcopenshell.entity_instance]:
     """
     Retrieves all subelements of an element based on the spatial decomposition
     hierarchy. This includes all subspaces and elements contained in subspaces,
@@ -966,29 +966,29 @@ def get_decomposition(element: ifcopenshell.entity_instance, is_recursive=True)
         decomposition = ifcopenshell.util.element.get_decomposition(element)
     """
     queue = [element]
-    results = []
+    results = set()
     while queue:
         element = queue.pop()
         for rel in getattr(element, "ContainsElements", []):
             related = rel.RelatedElements
             queue.extend(related)
-            results.extend(related)
+            results.update(related)
         for rel in getattr(element, "IsDecomposedBy", []):
             related = rel.RelatedObjects
             queue.extend(related)
-            results.extend(related)
+            results.update(related)
         for rel in getattr(element, "HasOpenings", []):
             related = rel.RelatedOpeningElement
             queue.append(related)
-            results.append(related)
+            results.add(related)
         for rel in getattr(element, "HasFillings", []):
             related = rel.RelatedBuildingElement
             queue.append(related)
-            results.append(related)
+            results.add(related)
         for rel in getattr(element, "IsNestedBy", []):
             related = rel.RelatedObjects
             queue.extend(related)
-            results.extend(related)
+            results.update(related)
         if not is_recursive:
             break
     return results