pop or imap accounts? What AV software are you using?

I have exactly the same problem and it's POP accounts, Happened around the same time, 3-4 weeks ago after I applied a Thunderbird update.

Seems I'm not alone, I am seeing a few reports of the same or similar problems in the last few weeks.

I have 2 emaIL accounts and both have the same problem with duplicates, one account is POP3 and the other is IMAP and both are getting thousands of duplicate emails. I have run the "Remove Duplicates" add-in over & over as well as the Repair Folder command, but the files are immediately replaced. I current have over 40,000 unread emails in my 2 accounts and I think it's a major problem. I tried to reduce the problem by using Message Filters to send some email sources to Trash but it's hardly haling/. I hope someone is working on a fix for this but i haven't seen any signs that they are. Maybe someone can tell me when they get a fix?, because the Remove Duplicate add-in is not getting it done?