The report fails with the following validation error:

QE-DEF-0459 CCLException 
RQP-DEF-0149 The query specification is incorrect. 
RQP-DEF-0191 Column names must be unique. Column name 'UnUsed' is used more than once.

This is an error appears in Cognos despite the fact that the column reported in the error message ("UnUsed") is unique, and not used more than once in the underlying SQL query. The root cause of this Cognos issue is unknown. 

Workaround

Edit the report in Report Studio, and save the report again.

If the error still appears, remove the column from the report, add it again to the report, and save the report again.