Friends fans left shocked after only just realizing who played Phoebe's twin sister Ursula

It might have ended over 21 years ago, but Friends has cemented itself among the most iconic sitcoms of all time.

The hit TV series, created by David Crane and Marta Kauffman, ran for 10 seasons on NBC between 1994 and 2004.

Starring Jennifer Aniston, David Schwimmer, Courteney Cox, Matthew Perry, Matt LeBlanc and Lisa Kudrow as Rachel, Ross, Monica, Chandler, Joey and Phoebe, it follows the chaotic personal lives of six 20-somethings in New York City.

A five-year stint on Netflix in the US and Canada between 2015 and 2020 spawned a new legion of dedicated fans - many of whom would've still been in diapers as the show came to an emotional end.

No matter how many times you've rewatched Friends, though, there'll always be something new to learn about the popular show.

Friends ran for 10 seasons between 1994 and 2004 (Reisig and Taylor/NBCU Photo Bank/NBCUniversal via Getty Images via Getty Images)

Some fans are only just finding out that Phoebe's identical twin sister Ursula started out as a character on another TV show - also played by Lisa Kudrow.

Ursula was a waitress on the sitcom Mad About You, which ran from 1992 to 1999 and therefore overlapped with Friends.

She only appeared in eight Friends episodes (versus 24 in Mad About You) and never failed to wreak havoc on Phoebe's life - like launching a career as an adult industry star under her name...

While Lisa played both Phoebe and Ursula on-screen, not many people realize that off-screen, she wasn't shooting those scenes alone.

In fact, it was Lisa's own sister Helene who was enlisted as Ursula's (and, sometimes, Phoebe's) stand-in.

The sisters aren't twins in real life, with Helene being four years older than Lisa. However, the two looked strikingly similar in pictures from the 90s.

Taking to social media, fans have been expressing their shock over the realization.

Over on Reddit, one fan said: "I caught an old picture of [Lisa and Helene] the other day from about the right era, and honestly couldn’t tell who was who."

A second agreed: "While not identical, they are very, very similar looking. Wow!!"

Over on X, formerly Twitter, another fan said: "Wait for real?" as another wrote: "I never knew this!"

Some TV magic was used to splice scenes of Phoebe and Ursula together, including the classic split screen method where two frames are overlapped to give the illusion the sisters are stood next to each other.

Lisa Kudrow played both Phoebe and her identical twin Ursula, but her real-life sister stood in as her body double (NBC)

So while Lisa was shooting both parts, for the scenes to appear seamless, she had Helene play as the double.

In an interview with Entertainment Weekly in 2019, Friends director Kevin S. Bright revealed Lisa 'didn't have a good time' shooting these particular scenes.

He explained: "She did not like acting with a double, and in a way she might have made it more difficult for herself because her double was her actual sister, [Helene].

"I think feeling the [stress] she put her sister into by being the double was more in her head at the time, so those scenes were a little bit tricky to shoot. But it ended up being a lot of fun when you put it together."

You'd never have guessed!