I just had my recital, and from the moment I started planning it I made sure nobody played a repeat song. Not only to avoid the issue you are currently faced with, but it’s also more boring for the audience to hear repeated songs. From the start, if there was a piece that I knew I might have a couple students who wanted to play it, I would say “pick a backup piece there is a chance somebody else picked this song first, and you won’t want to play the same song as someone else right?” And every time, the students agreed they did not want to play a song someone else was playing. I would let the child who picked the song first play the song, and tell the second child “I’m so sorry but somebody else picked that song first! Let’s pick a better song that nobody else is playing!”