I guess wait a month https://github.com/eyalroz/removedupes/issues/223

But seriously I have had issues with duplicate message twice in 20 years and that was when I was merging data from several old mbox files into a single one. Why anyone would need such a tool in the month post release of a new version in an IMAP world I really do not understand. Perhaps you have other issues that need to be looked at.

"Why anyone would need such a tool in the month post release of a new version in an IMAP world I really do not understand."

This comment is not helpful and it's borderline rude.

Try v. 0.6.0 of this add-on:

https://addons.thunderbird.net/en-us/thunderbird/addon/removedupes/versions/

Thanks! I wonder why I couldn't find that yesterday! Now, pardon me for being daft, but I cannot figure out how to start using this add-on and this page is not helping me much:

https://github.com/eyalroz/removedupes/?tab=readme-ov-file

Once the add-on is installed, access it from the folder context menu (Remove Duplicates...). There is a FAQ link on the github page.

Thanks I just figured it out!