Rated 4 out of 5 stars
Works pretty well, haven't had any issues yet and removed a ton of duplicates (especially in one of my RSS feeds). The only thing I've found thus far that I don't like is that the it's a little bit of a pain to setup. Perhaps adding a button with a dropdown menu that provides options. Included in that could be an option to scan all folders (since I don't keep all my email in one global folder I have to select the different main folders for each account).This review is for a previous version of the add-on (0.3.5).
You could just select the single 'Account Folder', the parent of the account's main folders (Inbox, Sent, etc.) - that works too.
As for a button - if every extension put its options in a button you would have endless clutter...
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
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.