When the input files contain records with equal keys, those
    records may not maintain the same order that they appeared in
    the input file. Specifying the /STABLE qualifier arranges records
    with equal keys in the order of the input files on output. If you
    use this qualifier when sorting multiple input files, on output,
    records with equal keys in the first file precede those from the
    second file and so on.

    The /STABLE and /NODUPLICATES qualifiers are mutually exclusive.