"Some singers are more jealous than others"
An interesting example of this was when Walter Pardon was approached by a local singer and told that he shouldn't give his songs to other people because when you did this they were no longer your own.
His (typical) response was "They're not my songs, they're everybodys".
The practice of not singing another singer's songs was very much a two-way street. Singers would never sing another's songs in their presence, or, only when persuaded to do so, and if they did, and had altered them in any way, they would apologise for having done so.
On the other hand, there is a story of a well-known revival singer who was booked at a Manchester club where one of the residents deliberately opened the evening with five songs which were known as being part of the guest's usual repertoire.
The guest sang them all again.
Jim Carroll