Review for Remove Duplicate Messages by Salvatore Randazzo
Rated 5 out of 5 stars
It works properly and I use it successfully since 1 day. My colleagues started using it too.
Today I had a problem. The addon correcly identified many duplicated messages, tagged them for delete, but the the delete process didn't work. I had to close Thunderbird and when I opened it again, the add-on worked smoothly.
To create your own collections, you must have a Mozilla Add-ons account.