Season 1: Episode 17: The One With Two Parts, Part II
Monica brings Rachel to the hospital, but Rachel has no health insurance so against Monica's better judgement, the two switch identities. When they have a double date with two cute doctors, they must continue the charade and end up getting into a fight. The gang throws Phoebe a party, but she is disappointed when she finds out Joey went to meet Ursula instead. When Phoebe learns Ursula stood Joey up, she confronts her sister who admits she doesn't want to date Joey any more and isn't returning his calls. Phoebe dresses up as Ursula and gives Joey an ultimatum: he can choose to go out with her/Ursula or remain friends with Phoebe. He chooses Phoebe, kisses Ursula/Phoebe... then realizes it's Phoebe. And, when Marcel swallows a Scrabble tile, Ross pulls through like any good father would.