Set items saved twice - keeps on resetting!

Hi, a long-time Thunderbird user here using 140.11.1esr (64-bit) Thunderbird for Linux Mintmint-001 - 1.0 with e-mail set up as IMAP for a hotmail.com e-mail. I'm having (and always have tbh) had problems with saving only one copy of "Sent" e-mails and it's driving me scatty. In "Thunderbird setup" for my hotmail.com account, I always de-select "When sending messages, automatically Place a copy in "Sent" folder" as I know my Hotmail account does that and then Thunderbird will pick that up to store locally as part of the IMAP syncing. I get, as expected, only one "Saved" copy which is fine. Then, for no apparent reason, I suddenly start to get duplicates even though the "de-select" is still there in Thunderbird setup for the account. The cure seems to be to select it as if I want 2 copies and send myself a test e-mail, then de-select it again so I only get one at which point I only get one copy again. It then works fine for weeks (or months!!!) then, bang, 2 copies start appearing again with nothing (seemingly) changed. I also have a gmail.com account set up as IMAP which never changes - I've always only ever got one "Sent" copy as expected. Clearly, this points to some sort of weird Thunderbird/Hotmail config issue which is odd as the "mechanics" of Thunderbirds internal handling of copies should not be affected by the remote e-mail and yet the issue is with hotmail.com but never with gmail.com accounts. As a Comms/Radar engineer it's really got me! Anyone else seen this and, if so, are you able to tell me what's up? What am I doing wrong? It's very odd. Cheers!