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

quote from webpage:

About this Add-on (As Thunderbird is undergoing massive changes under the hood, always look for a beta of this extension for support of the latest Thunderbird version. You are also welcome to report bugs or write me.)

Check you are using the correct version which works with the correct TB version. Do you need to update the addon ?

At the link above. Those bold sections are links to the author. Please tell the author: what OS you use. The version of thunderbird you use. The version of the addon. What you did and what happened.

Also post the same info in this forum question.

ARE you talking about POP or IMAP mail account folders? Are duplicates actually being downloaded or are they being resurrected? Are they occuring in the Inbox or other folders ?

Do you use a product like CCleaner ? If yes, do not let it have any access to Thunderbird files and folders as it can delete needed files. eg: Deleting of 'popstate.dat' file can cause repeat downloads.

Do you compact folders on a regular basis? If no, then move all wanted good emails out of Inbox. Delete anything you do not want, when Inbox is empty, right click on Inbox and select 'Compact.

You do not usually need to empty folder before compacting, but if you have a corrupted folder than it would be safer.

Hi Toad Hall

Thanks for your email. To answer your questions: what OS you use. Windows 10 The version of thunderbird you use. 52.9.1 (32-bit) The version of the addon. 0.3.17b1 What you did and what happened: I tried using the add-on with 100 highlighted messages, 50 of which were dupes. Under tools i selected Delete Duplicate Messages. A window pops up saying "No Duplicates found."

ARE you talking about POP or IMAP mail account folders? POP

Are duplicates actually being downloaded or are they being resurrected? Are they occuring in the Inbox or other folders ? Only in the inbox, to me it seems they're downloaded but I can't be sure if TB is creating them or what...there are no duplicates in the Sent folder, just the Inbox.

Do you use a product like CCleaner ? No, but the popstate-#.dat folders keep reappearing no matter how many times I delete them.

Do you compact folders on a regular basis? I do sometimes, but not regularly. I can try your suggestions of moving the non-dupe emails out of the inbox, delete the rest, and try compacting.

I think to issue here might be more in line with what anti virus product do you use?

In the interim I suggest you create an exemption in your anti virus product for the Thunderbird profile folder and it's sub folders. (there is a link in the troubleshooting information on the help menu that will open explorer opened in your profile folder to make locating is easier.

popstate.dat "folders" generally occur when an anti virus is messing with the profile folder during a download. the popstate.dat is a file in the file system Thunderbird uses to remember what mails have been downloaded. That they appear in the user interface as a folder usually points to the anti virus product in use. (although streaming backups and remote profiles are also culprits)