Review for Remove Duplicate Messages by longInt
Rated 3 out of 5 stars
Do not use this extension with IMAP when having many duplicates. It will crash your TB and MailServer as well. Go for the alternate version instead and use the permanently delete option. Works fine.
This version also seems not to find empty duplicate mails, the alternate version does!
To create your own collections, you must have a Mozilla Add-ons account.