A duplicates sub-index may not be altered

  Explanation:  You attempted to ALTER a sub-index which has  been
  defined  to  manage duplicates within an INDEX ONLY index.  This
  is not permitted.

  User Action:  If you wish to ALTER the sub-index, then you  must
  drop  the  INDEX  ONLY  index,  and  recreate  the index and the
  sub-index.