revert

diff --git a/lib/sqlalchemy/orm/clsregistry.py b/lib/sqlalchemy/orm/clsregistry.py
index 4c9e38b6bf6294df4821da40fa971730e4d1ad1a..ed5e59bbfcf51f50956b110d6f07e3490db1e917 100644 (file)
--- a/lib/sqlalchemy/orm/clsregistry.py
+++ b/lib/sqlalchemy/orm/clsregistry.py
@@ -69,11 +69,11 @@ def _add_class(
 
     """
     if classname in decl_class_registry:
-        if not isinstance(
-            decl_class_registry[classname], _MultipleClassMarker
-        ):
-            decl_class_registry[classname] = _MultipleClassMarker(
-                [cls, cast("Type[Any]", decl_class_registry[classname])]
+        # class already exists.
+        existing = decl_class_registry[classname]
+        if not isinstance(existing, _MultipleClassMarker):
+            existing = decl_class_registry[classname] = _MultipleClassMarker(
+                [cls, cast("Type[Any]", existing)]
             )
     else:
         decl_class_registry[classname] = cls