I try to not duplicate songs for each show, so most of my regulars are coming in early to reserve the song, be it picture, or summer nights, leather & lace, mockingbird, etc.
I think it keeps the show from turning into a contest, and I do hate contests!
If at the beginning of the show the song was done, I explain that if the new singer wants to wait around until the end of the show, I will allow them to repeat a song. ( usually they don't hang around that long, or I can get them to put in a few other songs that haven't been sung yet! And the original singer has left the building)
Rick
Rikki Tikki Karaoke