There is an add-on for this purpose:

https://addons.thunderbird.net/en-US/thunderbird/addon/remove-duplicate-messages-alte/

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

Thanks.

I've downloaded and installed this add-on but can't see how to run it. It doesn't seem to run automatically as I can still see the duplicated messages in the Trash folder. Am I doing something wrong here please?

Regards and TIA

Phil

After installation, restart TB, then right-click a folder, Remove duplicates... The are more settings under Tools/Add-on Options.

Many thanks again.

I don't think it removed 100% of the duplicates but it was probably greater than 90% and saved a lot of disk space, so I am well pleased.

Regards

Phil

Having the same issue. I have installed the Alternative one, but it doesn't appear to work. You can see my settings in the attached image.

I have probably 8000 duplicate emails in my Sent folder, all with duplicate messageIDs.

If I'm in the Send folder when I right-click on the folder in the left pane, and select Remove Duplicates, it runs for less than a second, and reports "Duplicate message search complete."

If I'm in another folder when I right-click on the Sent folder in the left pane and select Remove Duplicates, I get a [JavaScript Application] pop-up alert "Not searching special folders." ... which is wrong, since I unchecked the "Skip Special Folders" checkbox.

Thanks

Ok. My bad. I guess it did find the duplicate emails, after all. There was a pop-up that was behind, showing the 9,275 duplicate emails it found. However, when I clicked on Ok to move everything into the trash, nothing happened. However, I didn't have Thunderbird open when I clicked Ok.

So I started TB up again and clicked Remove Duplicates again and Thunderbird locked up. Once it started working again, I saw the pop-up behind. Clicked Ok and now it's re-downloading the 9298 messages in Sent.

So, I guess it worked. Will know when it completes the download. Currently stuck at "Downloading message 279 of 9298 in Sent..."

That said... I'm looking at the webmail interface for my email, and all the duplicates are still in the Sent folder.

"Downloading message 930 ..."

If downloading your Sent folder is slow, it could be caused by a security program scanning the folder. That should always be disabled (keep the real-time background scan) anyways, by adding the profile folder as an exclusion or exception, or disabling AV 'mail shield'.

If you are getting duplicates in your Sent folder, keep in mind that providers such as gmail, outlook.com, Yahoo, AOL etc. automatically copy sent messages to Sent, so you should uncheck 'Place a copy in' in Copies & Folders in Account Settings.

This is on my own email server running SmarterMail, and the duplicates appeared after I restored them from the Trash.

Currently manually moving all 2016-2018 emails in my Sent mailbox to /Archives/2016/Sent, etc, to see if the Remove Duplicates add-on works there.