public VersionTag setPosDup(boolean flag) {
    if (flag) {
      setBits(BITS_POSDUP);
    } else {
      clearBits(~BITS_POSDUP);
    }
    return this;
  }
sets the possible-duplicate flag for this tag. When a tag has this bit it means that the cache
had seen the operation that was being applied to it and plucked out the current version stamp
to use in propagating the event to other members and clients. A member receiving this event
should not allow duplicate application of the event to the cache.
geode-core/src/main/java/org/apache/geode/internal/cache/versions/VersionTag.java