Review for Remove Duplicate Messages by giantslor
Rated 5 out of 5 stars
I was skeptical, but it worked brilliantly. I had to go into the options and set a minimum of variables before it worked for me, but after that it was superb -- very fast and thorough in all stages. Worked great with Gmail IMAP. Allowed me to delete the larger message, an option which the supposedly rewritten "alternate" version doesn't offer.
To create your own collections, you must have a Mozilla Add-ons account.