Review for Remove Duplicate Messages by Martijn
Rated 4 out of 5 stars
Worked well for me. I especially love the 'remove directly' option, which solved my being unable to delete duplicates because I was over quota.
If I may request anything it would be more visual feedback. When searching 30k IMAP messages I couldn't tell the difference between a hang and a running search.
To create your own collections, you must have a Mozilla Add-ons account.