Accusations of joke stealing are as old as comedy. Sometimes, they are accurate. Milton Berle famously quipped that Bob Hope didn't have anything he didn't have a week later. I feel most of these cases are unintentional. Comedians generally think alike and notice the same things. It's not surprising that two or more comics would come up with similar jokes. In my own experience, I've had to eliminate jokes that I felt were too similar to other comedians. I think this is generally the best policy. Sometimes a joke can be altered enough to make it sufficiently different again, but I prefer to discard them as it forces me to write new material.