you can use this addon extension: https://addons.thunderbird.net/en-US/thunderbird/addon/remove-duplicate-messages-alte/

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

a. Refused to do the Sent file which was piling up 300 more dupes every time I polled gmail

b. When I move all of them it found like 2 dupes when there 1300 or so.

c. Not really approved by Thunderbird which I have a zillion emails going back 20 years. DUMP this add-on, dump Thunderbird as well. I hope in future to be able to decipher my life.

I'm having an issue using TB in that add-on to delete duplicate files has stopped working. I've highlighted about 1100 conversations that I can plainly see are duplicated, but when I run the add-on it tells me there are no duplicates.

Up til now the add-on worked fine.

I'm also curious why I'm getting dupes in the first place?

thanks

In my case, I'm getting occasional stretches of duplicate in-box message records. I use Thunderbird access to a small town ISP that has been playing with different support contracts for wider internet access. I use both wired to a router, and wireless networking and check my mail from three different workstations that all run Thunderbird and usually keep me "up to date" on read and unread pointers. Recently my most commonly used instance of Thunderbird has been showing occasional patches of dups, and the recommended solution software does not appear to me working on my self-maintaining very current versions of Windows 10 and Thunderbird.

Why would this problem "come and go" as I sign into mail from different locations, but always the same credentials and hosts?

I just noticed that my Thunderbird cache set at 350 mb was "full" and wonder if this has anything to do with my problems?