It's rude to hi-jack your friend's joke entirely.
Steve starts telling a joke at a party that his friend, Chris, already knows.
Chris wants to contribute to the joke.
Chris lets Steve finish telling the joke and joins in the raucous laughter even though he'd heard it already.
Chris starts talking over Steve to tell the rest of the joke.
Steve feels glad that Chris found the joke funny even though he'd heard it before.
Steve feels annoyed that Chris wouldn't let him tell his own joke.