A piano player and a singer would like to perform a song together. A piano player knows the song they like ("Memory" from "Cats - the Musical", to be precise). However it is a complete piano piece intended to sound alone, not just accompaniment part. There is a right hand part and the left hand part, both have multiple notes sounding together at times.
If we then add a singer to this performance, is it necessary to modify the song trying to remove existing melody line, or it is ok to sing the sounding piano notes at the same time?