Allow user to toggle duplicates when getting inverses. See #1747.

src/ifcopenshell-python/ifcopenshell/file.py

@@ -378,7 +378,7 @@ class file(object):
 
         return [entity_instance(e, self) for e in fn(inst.wrapped_data, max_levels)]
 
-    def get_inverse(self, inst):
+    def get_inverse(self, inst, allow_duplicate=False):
         """Return a list of entities that reference this entity
 
         :param inst: The entity instance to get inverse relationships
@@ -386,7 +386,10 @@ class file(object):
         :returns: A list of ifcopenshell.entity_instance.entity_instance objects
         :rtype: list
         """
-        return [entity_instance(e, self) for e in self.wrapped_data.get_inverse(inst.wrapped_data)]
+        inverses = [entity_instance(e, self) for e in self.wrapped_data.get_inverse(inst.wrapped_data)]
+        if allow_duplicate:
+            return inverses
+        return set(inverses)
 
     def remove(self, inst):
         """Deletes an IFC object in the file.

src/ifcopenshell-python/test/test_file.py

@@ -191,7 +191,12 @@ class TestFile(test.bootstrap.IFC4):
     def test_getting_inverse_references_of_an_element(self):
         owner = self.file.createIfcOwnerHistory()
         element = self.file.createIfcWall(OwnerHistory=owner)
-        assert self.file.get_inverse(owner) == [element]
+        assert self.file.get_inverse(owner) == {element}
+
+    def test_getting_multiple_inverses_if_an_element_is_referenced_twice_by_the_same_element(self):
+        user = self.file.createIfcPersonAndOrganization()
+        owner = self.file.createIfcOwnerHistory(OwningUser=user, LastModifyingUser=user)
+        assert self.file.get_inverse(user, allow_duplicate=True) == [owner, owner]
 
     def test_removing_an_element(self):
         element = self.file.createIfcWall(GlobalId="global_id")