Intersting comments about pinching the guest's songs when they appear at a club. Not always that straight forward . .
If you have never seen a particular artist before, nor purchased their latest album, can you be forgiven for inadvertently singing a traditional song that by coincidence turns about to be their party piece?
People tend to support their local clubs regardless of who the guest may be. I have certainly attended several nights in the last year where the guest was 'new' to me. Excellent nights most of them have been too. I'd like to apologise here if I have sung a song I shouldn't have. But I suspect that most of said artists have a sufficiently broad repertoire to be able to adapt their set!
And there is no reason for a guest to suppose that a traditional song sung by a floorsinger comes from that guest's own recording. Many a lesser singer will have learned their songs from the same limited sources as the guest. Hence the similarities.

I guess it is always unfortunate when the situation occurs. Possibly uncomfortable for the rest of the audience too, who may know the score. But it shouldn't be always assumed that there was a deliberately undermining choice made.