Why Does Thunderbird Show Both “Sent” and “Sent Items” for Microsoft IMAP?

I’m using Thunderbird to access an organizational Microsoft mail account via IMAP (outlook.office365.com).

Recently, I’ve noticed two folders containing sent mail:

1. Sent
2. Sent Items

Both are marked with the green paperpale icon in the TB folders view. One of them has ~970 messages, the other has ~1350 messages. The folder with more messages has lots of sets pairs of duplicate messages; and in fact, over a longer while I had noticed that somehow, pairs of dupes form my Sent mail folder.

Morever, when I send a message, TB gets stuck in the phase of place a copy in the Sent folder, and I have to cancel the attempt. A copy is placed in the “Sent Items” folder though.

What could be the cause of this situation? Is this bifrucation “real”, or some phantom? And – what should I do, if anything, unify these two folders into one?