This may be a little complicated to implement, and hard drives are generally large enough nowadays that this isn't a huge concern (although it certainly makes for some cluttered folders) but I've noticed that if you have auto-save replays on, load a replay, and play it to completion, an identical replay gets saved.
I would suggest that, if a replay is loaded (from file rather than from backwards frame step), don't autosave a new replay unless new actions are taken.
Makes sense to me. Will look into this, should be simple. (If all else fails, it could be done by CRC'ing the replay at load time and again before autosaving, and skipping the autosave if they match.)
Implemented for next update. :)