Review for Remove Duplicate Messages by Seattle
Rated 4 out of 5 stars
Unfortunately checkboxes for keeping messages still don't show up in the newest 0.61 version with my Thunderbird version of 52.8.
Otherwise it seems to work fine - finds duplicates across many folders.
A fix would be great, because now I have to delete the found copies manually in the folders.
To create your own collections, you must have a Mozilla Add-ons account.