Don't duplicate the keras path (or any path).

PiperOrigin-RevId: 442754247

Author: MarkDaoust

tools/tensorflow_docs/api_generator/doc_generator_visitor.py

@@ -131,6 +131,8 @@ def __setitem__(self, path: ApiPath, obj: Any):
       path: A tuple of strings.
       obj: The python object.
     """
+    assert path not in self
+
     parent_path = path[:-1]
     parent = self[parent_path]