There is a definite protocol here, certainly this side of the pond.
If you are a regular at a club and another regular regularly sings a certain song then you respect that and leave him/her to it.

If that person ceases to sing the song, leaves the area, or dies then that song is surely'up for grabs'.

There are thousands and thousands of songs. Surely we're not that desperate that we have to sing one song. Even those singers (some on Mudcat) who try to sing a different set of songs every week at the club can manage it without taking someone else's song in their club.

Visitors of course, as has been stated, cannot be expected to know every song that is regularly sung at the club, so common sense rules hopefully.

Similarly at tune sessions, if I arrive late I will wait a considerable length of time before starting up a set in case those tunes have already been played.