When extend a view object , it shows Each row in the Query Result Columns must be mapped to a unique

 I need to add a column to a view object

 the progress: create a new view object that extends the parent view object(expert mode) , then  update the SQL,and  it tests validly, but when step four Attribute Mapping it show me :Each row in the Query Result Columns must be mapped to a unique Query Attribute in the mapped Entity column.

What's the problem? Thank you in advance for any help you can offer.