Specifies that duplicates are compressed. If a sorted ranked
    index allows duplicate entries, you can store many more records
    in a small space when you compress duplicates, therefore,
    minimizing I/O and increasing performance. Oracle Rdb uses
    patented technology called byte-aligned bitmap compression
    to represent the dbkeys for the duplicate entries instead of
    chaining the duplicate entries together with uncompressed dbkeys.

    Duplicates are compressed by default if you specify RANKED
    without specifying the DUPLICATES ARE COMPRESSED clause.

    You cannot use the DUPLICATES ARE COMPRESSED clause when you
    create nonranked indexes or when you specify the UNIQUE keyword.

    See the Oracle Rdb Guide to Database Design and Definition for
    more information on sorted ranked B-tree indexes.