Contestant Maggie, 70, looked and sounded incredibly similar to well-spoken kitchen guru Leith, 82, leading to a running joke that the judge said she was already getting sick of.
Read more: Who are the Bake Off 2021 contestants?
As Leith and her fellow judge Paul Hollywood approached Maggie's baking bench for the first time, Hollywood began the joke by repeatedly calling the baker Prue and asking: "Were you separated at birth?"
Maggie and Leith shared a laugh about the chocolate content of her Swiss rolls and frowned upon the fact that Leith husband preferred "adulterated" milk chocolate to dark, leading host Noel Fielding to point out their similarities further.
Listening to their very similar accents, he told them: "I'm loving listening to you and Prue chat, I've never felt more like a chimney sweep in all my life. It's like a Dickens novel."
But by the time the judges and hosts gathered to discuss how the bakers were doing at the end of the second challenge, Leith had already tired of quips about her double.
With Hollywood pretending to confuse their names again, Leith asked if it was going to be a joke that would run for 10 weeks.
She said: "I'm beginning to hope Maggie goes out just to get rid of this joke."
Viewers agreed that the likeness was uncanny.
One person tweeted: "If Maggie steals Prue's glasses she can take the internal espionage route to winning this."
Someone else added: "I expect by next week Davina’s confirmed Prue and Maggie as long lost sisters."
Another viewer agreed: "Prue and Maggie talking to each other is like an episode of the Crown."
Read more: Who are the Bake Off judges and presenters?
The show's fans were delighted to see its return to Channel 4 for series 12, with German baker Jurgen also proving a hit for his dry sense of humour.
One fan tweeted: "Jurgen! Early favourite and low key hilarious, either by choice or accident."
Another person commented: "Jurgen is already my winner and I will hear no other arguments."
The Bake Off series opener tickled fans as Hollywood, Leith, Fielding and his co-host Matt Lucas donned country music outfits to perform a version of Billy Ray Cyrus hit Achy Breaky Heart, retitled as Flaky Pastry Tart and complete with cake-themed lyrics.
A fake-tanned Hollywood posed as lead singer while Leith, on keyboard, sported a large moustache.
Watch: Paul Hollywood and Prue Leith decide who's more intimidating