Nina Dobrev as Elena Gilbert & Katherine Pierce in 'The Vampire Diaries'
Don't let the taint of teen bloodsuckers fool you. Dobrev is tearing it up over at the 'Diaries' set, and is often the best two things about the show -- and that's saying something since it's actually pretty awesome. It helps that unlike certain other teenage vampire lovers, Elena isn't auditioning to be a doormat, and Katherine is a lusty, brainy vampire with a back story straight out of Tolstoy. Big bonus points to Dobrev; it is impossible to mix up which character she is playing, regardless of the costume and set cues. She transforms herself.
Lisa Kudrow as Ursula Buffay & Phoebe Buffay in 'Friends'
It's sort of startling in hindsight, but Ursula is the much older twin. She was created by 'Mad About You.' When Kudrow was cast on 'Friends,' making her character into Ursula's twin was a joke on the part of the writers. The really funny thing is the way Ursula's occasional visits to the 'Friends' version of Manhattan highlighted the borderline aspect of Phoebe's personality. Usually sweet, silly and distracted, Phoebe could also be cold, acerbic and mean. Ursula was a reminder of how thin the line is between funny and cruel.
Elizabeth Montgomery as Samantha & Serena in 'Bewitched'
Samantha's counter-culture cousin Serena really should have had her own spin-off. She couldn't have been a more perfect emblem for the times, being the only member of Samantha's family that supported her "mixed" marriage and who occasionally dated non-warlocks herself. The visual effects here consisted of a fake beauty mark, a black wig and a few miniskirts, but the reruns had me fooled when I was young. I remember wondering how they found someone who looked so much like Samantha to play her cousin, proving that kids are weird: they may understand how Hollywood casting works, but not simple disguises and camera tricks.
Barbara Eden as Jeannie & Sister Jeannie in 'I Dream of Jeannie'
A blatant rip-off of the Samantha/Serena gambit in 'Bewitched,' Jeannie had to have a dark-haired version of herself too. Played with a wink and a nod, this evil twin doesn't even get her own name -- she is also Jeannie, and the main difference between her and her sister (besides being a brunette) is that she has more than one outfit. I'm sure the writers and creators would try to convince you that Sister Jeannie was more sneaky or sensual or liberated, but considering the original Jeannie was all of those things too, i say weak sauce.
Sheryl Lee as Laura Palmer & Maddy Ferguson in 'Twin Peaks'
I've always suspected the late arrival of Laura Palmer's lovely cousin Maddy was due to a realization by David Lynch that he'd killed off one of his most exciting actors before the show even started. Lee's angelic face was so iconic and her scenes in flashback were what kept this bizarre series rooted in a human realm. Doubles are one of Lynch's pet themes and he often creates complex roles for the actresses he works with. Lee here is no exception. Next time you re-watch, check out how slowly and subtly Maddy's behavior shifts over her time on 'Twin Peaks.'
Emily Rose as Audrey Parker & Lucy Parker in 'Haven'
This show upped the ante for Rose and for fans as Audrey's identity crisis gets more intense. It's one thing to find out you might look exactly like your mom, and be the cause of all the paranormal activity you've been tasked to investigate, but it's another to find out you may not even be who you think you are.
Grace Park as Sharon Valerii & Sharon Agathon in 'Battlestar Galactica'
If Grace Park had a plan for portraying the various Number 8s differently throughout this series, then it turns out like most Cylons, her plans were nebulous. That said, I often found Park's performance incredibly affecting and I'm not sure the plot or characterizations needed her to differentiate between the various versions of her character. They were, after all, clones. Looking back, what impresses me most is that Park managed to convey strength and weakness, bravery and cowardice, bitterness and openness so well in both characters and often, in the same scene.
Sarah Michelle Gellar as Buffy Summers & The Buffy Bot in 'Buffy the Vampire Slayer'
It's nice to remember that Gellar has experience playing different versions of herself onscreen. What started out as a small plot point became a central theme in this series, and Gellar spent a surprising amount of time onscreen playing a stiff, stilted and hilariously blunt fem-bot version of her tough-as-nails character. Like David Lynch -- though considerably less pretentious about it -- Joss Whedon loves to play with the idea of doubles, and the entire Scooby Gang had the chance to evil-twin it up, but the unexpected pathos evoked by Gellar as the Buffy Bot takes the cake. (Sorry, Vamp!Willow fans.)
Who else has doubled up on TV? And the entire cast of 'Star Trek' doesn't count! Tell us in the comments.