diff --git a/gitlab/base.py b/gitlab/base.py
index 5eb111851..53bf45237 100644
--- a/gitlab/base.py
+++ b/gitlab/base.py
@@ -132,7 +132,7 @@ def __ne__(self, other: object) -> bool:
         return super(RESTObject, self) != other
 
     def __dir__(self):
-        return super(RESTObject, self).__dir__() + list(self.attributes)
+        return super(RESTObject, self).__dir__() | self.attributes.keys()
 
     def __hash__(self) -> int:
         if not self.get_id():
diff --git a/gitlab/tests/test_base.py b/gitlab/tests/test_base.py
index a0adcb03d..6ca261d6c 100644
--- a/gitlab/tests/test_base.py
+++ b/gitlab/tests/test_base.py
@@ -135,6 +135,10 @@ def test_update_attrs_deleted(self, fake_manager):
         assert {"foo": "foo"} == obj._attrs
         assert {} == obj._updated_attrs
 
+    def test_dir_unique(self, fake_manager):
+        obj = FakeObject(fake_manager, {"manager": "foo"})
+        assert len(dir(obj)) == len(set(dir(obj)))
+
     def test_create_managers(self, fake_gitlab, fake_manager):
         class ObjectWithManager(FakeObject):
             _managers = (("fakes", "FakeManager"),)