In a rather lively place I go to where there's no defined order to sing in (it's up to you to take your turn), I've had ppl cut in with their own song 2 seconds after I've started a song. It's usually just bcos it takes 5-10 seconds for the room to quieten down for a singer though, and the other singer hasn't noticed that you've started! At least, the others are mostly nice ppl, so I choose to assume that's the reason... ;-) Cutting you dead mid-song is right out, though - sod 'em, there's other places to play, and make sure anyone you meet at other local sessions knows to avoid this one.

Graham.