I can't say I have every 'stolen' a kill but there have been many times I have teamed up to gang up on an aircraft. If I see two a/c interlocked in combat I will often fly above then and see who is winning, and will dive down to help out if I think one of my squaddies or teammates are in trouble. I suppose I might have jumped the gun one or two times but I guess who hasn't.
I know I have had a couple probably stolen from me, generally in big furballs when someone dives in while I am on the six of a fleeing aircraft. It is irritating but what are you going to do?