"I think it is good manners not to do a song that you know another is likely to sing."
But why would anyone want to sing the same songs every month? Or so often that it will naturally be assumed that Mr. 'A' will sing 'x' and Mr. 'B' will sing 'y'?
Isn't that boring both for yourself, and for your audience?
It's not as though there's a limited supply of songs going around...
I'm probably treading on gouty toes galore here, but although it may have been 'the way it's always been done' (which IMO has never been a satisfactory reasoning for anything whatever..), but irrespective of that, it seems almost as cliquey as 'my mug' and 'my chair' type syndrom.
And as for Guests not being welcome to perform songs which are repeatedly performed by regulars, I for one would find it more interesting to hear a fresh rendering of song 'x', than hear it sung yet again by Mr. 'x'.
As an outsider to regular folk clubs (though no longer a novice), this stuff atill makes no decent sense to me.