Would we mark a recording a cover for a duet even if one of the artist is the original artist while the other part is sung by another artist?
I’d probably let this depend on who is the main artist of the later recording. If it’s the original artist, then I wouldn’t consider it a cover, if it’s the other one then yes.
Yes something like @paulakreuzer.
But generally I just would not mark it as cover if any of the writer or original performers is here.
But what if it’s sung “unofficially” as in they sing it with a fan and not officially released?
That doesn’t make it a cover, just an “unofficial” recording (whatever that means).