Exit set ops when nonunique

pandas/core/ops/__init__.py

@@ -307,13 +307,14 @@ def should_reindex_frame_op(
  if not isinstance(right, ABCDataFrame):
  return False
 
+ if not left.columns.is_unique or not right.columns.is_unique:
+ return False
+
  if fill_value is None and level is None and axis is default_axis:
  # TODO: any other cases we should handle here?
  cols = left.columns.intersection(right.columns)
 
- if len(cols) and not (
- cols.equals(left.columns.unique()) and cols.equals(right.columns.unique())
- ):
+ if len(cols) and not (cols.equals(left.columns) and cols.equals(right.columns)):
  # TODO: is there a shortcut available when len(cols) == 0?
  return True