Lets get something clear comic to comic.
Anyone who knowingly steals another comics joke is something beneath a thief. It is the lowest thing one artist can do to another artist. A joke is intellectual property. A joke is less than a sentence but more than a collection of words. A joke is someone's individual and unique take on a subject. To take that and pass it off as your own is to be a traitor. A traitor to the belief that comedy can be an art. We might occasionally come up with similar jokes based on something in the news, or even a similar joke about something random, but stealing a persons joke completely and knowingly is low. Apologize, stop doing the joke and hope people who are in a position to hold you back don't care as much as other comics do. It's just plain wrong, man. It would be like stealing another man's horse in the old west. It's like taking the last condom from your buddy's wallet. It's like taking his wallet too! I know that the audience doesn't know or even care if it's not yours, but comics do. That small group of fellow artists that you will see for the rest of your career know. They don't forget shit like this. None of us do. It will follow you around and shadow every step of your career. We have all had the occasional riffing moment where a friends joke slips out. Or the conversation where the person says something you spit back out to laughter on stage. These are different, even forgivable instances. I am talking about out right theft. Someone else wrote it but someone else shouldn't be saying it.