Why, when Jamie and Fran see Phoebe in Central Perk, and assume it's Ursula, do they assume she works there? Jamie says that Ursula was in Riff's last night, and Phoebe certainly isn't doing anything that would imply she works there; she's not wearing an apron, nor carrying a tray with food or coffee, nor holding a pad and pencil and asking patrons for their orders. Even if they assume it's Ursula, why don't they simply assume Ursula is there as a patron?

I haven't watched enough Mad About You, myself. Do any of the episodes of that series mention Ursula having a twin? If they don't know about Phoebe, then the confusion in this episode makes sense.

Also, it should be mentioned that in an episode of Mad About You, Jamie and Paul see Ursula at Riff's and note how they haven't seen her in a long while. She mentions that she's just been hanging out with friends (or Friends, if you prefer).

Lee, their assumption that it's Ursula makes sense. Their assumption that she works at Central Perk does not.

I've never watched an episode of Mad About You myself, but I know that the twin thing came up on Friends first because both were on NBC, and they wanted to acknowledge the other show. I don't know if they made references on Mad About You, but if they did, it would've been only after they first did so on Friends.