what version of thunderbird are you using?

Version 146.0 (64 bit)

that's what I expected. I think the 146 release caused that. I have no true solution, but I suggest rightclicking on each folder and checking properties to verify that thunderbird thinks they're both assigned to the account. Also, check to see which one thunderbird is using for sent messages. Once you can verify the unneeded one, you may be able to rightclick and delete. But this may also be a case where this bug may be fixed in an upcoming release - but I don't know that. My guess is that your account may have a folder that the 146 update thought might be used for sent messages and decided to name it such. But the logic used escapes me. There have been several posts and folders misnamed.

Locking this as a duplicate of https://support.mozilla.org/en-US/questions/1556662