index definition contains an identical column reference in  both
  keys

  Explanation:  The index definition repeated  a  reference  to  a
  column  in  a  table  within  the  primary  index  key  and  the
  duplicates index key.  This usage is not allowed within an INDEX
  ONLY index definition which has duplicates sorted.

  User Action:  The index can  not  be  created  in  this  manner.
  Recreate  the  index  using  making  sure that the column values
  within  the  primary  index  key  are  not  reused  within   the
  duplicates index key.