/sys$common/syshlp/helplib.hlb MERGE, /DUPLICATES, Full Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    By default, Sort/Merge retains records with equal keys. The
    /NODUPLICATES qualifier eliminates all but one record with equal
    keys. The retained records may not appear in the same order as
    they appeared in the input file. If you want to specify which
    duplicate record to keep, invoke Sort at the program level and
    specify an equal-key routine.

    The /STABLE and the /NODUPLICATES qualifiers are mutually
    exclusive.