In Mad About You, Lisa Kudrow played a supporting character, a waitress named Ursula. The creators of Friends decided to make Ursula Phoebe’s twin sister.

In one of her interviews, Lisa Kudrow said, “When Friends got its time slot on Thursday night at 8:30 after Mad About You, everybody realized ’we need some explanation for the audience — why are they seeing the same girl with the same voice at 8 o’clock and 8:30 on 2 different shows?’”

By the way, Leila Kenzle and Helen Hunt appear on Friends in a scene where they think that Phoebe is Ursula. This is because they both were on Mad About You and they know Ursula from there.