does lisa kudrow play phoebe and ursula, ou does she really have a twin sister?

She plays both of them. When they are on the same screen they shoot the scene twice, when she is being Phoebe, a back up actor will be Ursala so Phoebe has someone to talk to, and then vice versa. They then combine the two shots into one

She does not have a twin. However, she does have an older sister and they do look alike!

The character Ursula is from the tv montrer Mad about you, which Lisa was on at the time, then since Friends and Mad about toi were in the same city they decided to put Ursula on Friends as well, so they Lisa played to different characters, a lot of célébrités have done that kind of acting, Lindsay Lohan in The Parent Trap and I know who killed me, Lucy Hale in an episode of Les Experts Miami, Nina Dobrev in The Vampire Diaries, Hilary Duff in The Lizzie MaGuire movie. It's a common thing that célébrités have done.