Would a duet be allowed?
In principle, sure thing. Someone else asked a similar question and here's basically what I said:
As for collaborations, in principle those are fine. I mean, if for example you'd sent me a whole submission of one artist that was from a collaboration with another artist, and it scored 8/10 or higher so got banned, and then sent me more of that second artist later on, I might reject that. But the odd guest appearance is fine!