Rated 3 out of 5 stars

Haven't actually tried it yet but I will later tonight. To be on the safe side I plan on following this process:
Do some cleanup
Delete all trash
Backup Email
Try the Remove Duplicates addon, make sure it's set to check the entire message and delete to trash.
Check Trash to see what it deleted and make sure the originals not only exist but, are unharmed.

If I like it I probably won't post another review, if I like it but, it lacks a feature I'll post a review with my suggestions.

Also: When I try it later tonight I will be offline so if there is a problem with new messages downloading during the process that should be negated.

This review is for a previous version of the add-on (0.3.5).