Update apply.py assertion message

Improved assertion message for possible duplicate column names

pandas/core/apply.py

@@ -651,7 +651,7 @@ def normalize_dictlike_arg(
             and any(is_list_like(v) for _, v in func.items())
         ) or (any(is_dict_like(v) for _, v in func.items())):
             # GH 15931 - deprecation of renaming keys
-            raise SpecificationError("nested renamer is not supported")
+            raise SpecificationError("nested renamer is not supported. Possible duplicate column names.")
 
         if obj.ndim != 1:
             # Check for missing columns on a frame