a nested index definition implies that duplicates are sorted

  Explanation:  An  INDEX  ONLY  index  must  be  unique  or  have
  duplicates  sorted.   If  a nested index definition exists, then
  the duplicates handling must be specified as sorted.   Likewise,
  if the duplicates handling is specified as sorted, then a nested
  index definition must exist.

  User Action:  The index can  not  be  created  in  this  manner.
  Recreate  the  index  as  a  unique  index,  or  with  a  nested
  duplicates index, specifying  that  the  duplicates  are  to  be
  sorted.