When you leave messages on the server, Thunderbird updates a local file (popstate.dat) with where it is up to with the downloads for that account. If Thunderbird terminates before that update occurs, or something on your PC like an anti virus or other software prevents the file from updating then the mail will be downloaded again.

My guess is you have your settings as leave on server until I delete, which would see the second copy deleted deleting the server copy and ending the problem. Perhaps check in your account settings in Server setting exactly what you have set there.

Try leaving Thunderbird running for 10 minutes or so after it gets messages and see if the issue reoccurs going forward.

I don't have "until I delete" set. But I didn't change any settings anywhere. The only thing that has changed is the update to version 102.0.

The problem keeps happening. I use Thunderbird the same way all the time. I don't end it before the emails are downloaded.

I remind you that this only happens with emails that are moved by the filter to a folder other than the Inbox. My guess is that Thuderbird doesn't write this anywhere and that's why it downloads it again and that it's a bug. I don't know where else to write so that the developers can check and fix it?

New knowledge: I turned off email downloads when starting Thunderbird and found that Thunderbird does not download the email again, but creates it again in the Inbox (even though the filter previously copied it to the specified folder).

To repeat: At the first launch, the email is downloaded and copied to the specified folder. At the second launch, the same email will be created in the Inbox, even if it is already in a different folder.

Previous versions did not behave this way.

I will try to create the filters again.

It looks like the issue was fixed in the 102.0.1 update:

FIXED Mailbox MSF files could become corrupt in some circumstances

That didn't solve it. About 5 times it was OK and then the problem appeared again.