Actually it is expected behavior for Gmail as well as Outlook accounts - also see this question. After all, an IMAP server should contain such a copy in order to be reached by other clients, so it makes sense in some way even though the local setting can point to some other local folder, even from another account.

If you view both sent messages’s sources (using ctrl-U), you will see one of them has shorter headers (the local copy), the other one will have longer ones as if it has passed the received server. Therefor I would just leave the Local copy setting in Thunderbird unticked - it will also save (double) storage and prevent additional delays.

I’m not aware of any other Thunderbird settings that may have been changed and lead to changed behavior - even "unsyncing" the Sent folder in Synchronization & Storage doesn’t change it - nor affecting bugs fixed recently. I haven’t checked for any related server settings using webmail.

Thanks for clarifying. An observed issue with this bahavior, ie when selecting copy Sent folder option, is that Outlook.com webmail keeps 2 copies of the same message in Sent. I would expect only one to be on the remote server.

As written in the other question, both Gmail and Outlook services put a copy of any sent message (using their SMTP server) into their Sent folder automatically, and so they will for any other client than Thunderbird (that may not do so of its own). That means doing similar for the clients themselves will serve no purpose, as well as lead to an additional message (= the local copy) being copied over to the server, hence containing a shorter header. I see the same, though I can’t explain why you didn’t see this behavior previously, unless that local Copy setting changed.

Do you think your issue is resolved?

Yes I see this resolved now knowing the behavior of TB with Outlook and Gmail for sent messages. Thanks.