I was at a concert once when some local people came on. They started singing- and a voice right behind me joined in with harmony. As it happened, the owner of the voice was good - but it was distracting and not what I had paid for.

I turned around and said, You are very good, but I want to hear THEM.

A tricky one is that. If the performer is singing a chorus song and invites the audience to join in, then it's OK. But sometimes I've had the same problem. There was one time when a bloke sitting near me in the audience clearly knew all the words to every song the guest did, and insisted on singing along, not particularly tunefully, either. I just put up with it, but maybe I should have asked him to belt up. Trouble was, he was bigger than me :-(