I recently came across an article about a Friends fanatic who spotted a mistake in the first episode of Friends.
The mistake is that Phoebe is shown at two places simultaneously. Was that a mistake or Phoebe's twin Ursula?
It's definitely a mistake.
There are tons of clips throughout the series (and pretty much every other TV series and movie, too) where characters are seemingly teleporting around because each scene is being shot multiple times, and then, later on, when the best take is being selected for each scene it may not line up with how the characters move around on the set, or what they do, or what items they may be holding, and so on.
It's also pretty clear that Phoebe and Ursula never hang out together, so it's definitely Phoebe.