Thunderbird recently updated itself to 91.2.1. I maintain 5 separate addresses for various separate purposes. Upon launching TB today it has been in the process of downloading, 5,416 duplicate e-mails going back to 05/02/2018 *** for the 1st account***. I keep the most recent 4 years of communications in my in and outboxes and at the close of each year, I archive the oldest year ...my most recent archived calendar year is 2017. The duplicates suddenly stopped on 1/19/21 at 6:50 pm, for some reason not duplicating the last 80 or so messages. As you scroll through the messages..... one shows as read, the other is bold text and unread.

I don't know what caused this. why it happened or how to safely delete the duplicates while saving the originals. Safe suggestions welcome