That song has to be somebody's favorite song. Everybody has favorite songs they could listen to all day long.
I inform the person turning in the repeat that the song has been sung. That gets several responses.
1. Were they any good?
2. Are they still here?
3. I can do it better.
4. How long ago?
5. Can i still sing it?
6. OK, i don't want to sing it after someone else.
7. I'll pick another song.
I tell them it doesn't matter to me. I'll play it again, if they want to sing it.
If someone comes up and wants to sing it because "They can sing it better," They usually want to do it back to back. In which case the answer is no. If the same song comes up in the rotation and they have other slips under it. I'll change their selection to the next song. (They usually come up and want to postpone it or delete it altogether anyway.) They can sing it later if they really want to. I just don't feel right about telling a singer that has come to partonize my gig, they can not sing the song they want to sing. They are paying for it. By their food and drink purchases. I won't stop anyone from singing the song they came to sing. It's only a song. They are not going to slash someones throat.