added option to keep the oldest duplicate (rather than the newest)

Added a new option to remove all but the _oldest_ duplicate to supplement the new option I added earlier to remove all but the _newest_ duplicate.  Let me know if you see any weird behavior as a result, but it should be stable.

Also, I couldn't think of a better shorthand command-line switch other than big O vs little o. Let me know if you think of something better.