https://support.mozilla.org/en-US/questions/1453812 reports the same issue

It sounds like you are using https://addons.thunderbird.net/en-US/thunderbird/addon/removedupes/ ?

If so, https://support.mozilla.org/en-US/questions/1453812 is not the same issue, at least with respect to removing duplicates.

Remove duplicates addon updates

I uninstalled the Remove Duplicate Messages add-on I updated the program to ver.128.2.0esr (64 bit) I installed the add-on. The option to remove duplicates still does not work - no duplicates found.

Download the correct version of the addon for TB 128 ( image 1) and install it. Then go to TB add-ons page Choose the add-on's wrench icon. Adjust the preferences as in icons (2+3)

I didn't download the add-on again, I just set the parameters provided by Lampros and the duplicate search worked. After deleting/moving to the trash, you still need to do some tidying up, otherwise the folder will have a duplicate size.

Thank you for your help