It happens, and it is a compliment, backhanded, and one you would rather be without, but a compliment.

How about this, though.
I had a 'friend' who, on hearing that I had 'discovered' a no-longer-performing duo, Flanders and Swann, said "I will get you their LP's at trade price." When I received them four weeks later, he had learned some of the best numbers, down to plagerising the intro's and performing them in our scene.
The records weren't even very cheap! He even did one of the #s on the night he handed them over, and he smiled at me.

In the situation where someone 'steals' your act, I had a couple of very powerful songs that I NEVER performed. They were unaccompanied, so didn't need regular accompaniment practise, but I considered them strong and loud enough to blow your socks off. Unaccompanied singers and monologuists always have something in reserve, and you can bet your regular listeners will know what is going on.