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.
