CS - don't know if it still happens in folk clubs, but it used to be wonderful when it did.
A singer would pick up and 'reply' to another's song, followed by a response from either the original singer, or another.... It can become laboured if carried on too long, but when it worked, you never forgot it.
We used to build themed evenings around subjects such as 'The Battle of The Sexes" in The Singers Club - still got the notes of one of these somewhere.
Jim Carroll