Local Drafts folder was slow then Thunderbird started resending all the emails in my sent folder
Sending an email slowed right down, so I saved it and tried to open it from the Drafts folder, a local folder. Drafts was agonizingly slow to open a draft. (I tried this several times to send a particular email.) I clicked the Repair Folder button for Drafts. Then I clicked the Repair Folder for my local Sent folder. Bad move. Suddenly Thunderbird started resending all the emails in my Sent folder. Starting in 2020.
I had to disable my internet connection, then change my Sent Folder back to a server folder and disable my SMTP settings for the moment. Ouch!
Thunderbird doesn't seem to work well with local folders any more.