Making Use of the Twin

If you want to pull out some kind of Doppelganger in an episode of TV, the ideal way to achieve it would of course be to bring in a whole other actor who just happens to look exactly like the actor you're duplicating. Unfortunately, actors generally don't have identical twins hanging around.

But hey, sometimes they do! Twins have a certain advantage when it comes to getting in movies and TV. For one, child labor laws limit the amount of filming a kid can do in a single day. When you've used one kid to the daily quota, there's always the backup kid. For another, identical twins make excellent doubles, which works even if the twin isn't an actor.

Not to be confused with Backup Twin. And the title is not referring to inciting a three way between twins. Come on, guys.

Anime & Manga

 * Ranma ½: Semi-example: when Angela Costain, the North American dub actress for Nabiki Tendo, wanted to attend flight school, her sound-alike sister Elaina Wotten-Costain took over the character for season six. Her performance was perfectly indistinguishable from her sister's -- many fans didn't notice the actress had changed

Film

 * Terminator 2: Judgment Day,
 * There is a shot where Sarah Connor appears on screen at the same time as the T-1000 is mimicking her. This shot was achieved using Linda Hamilton's twin sister Leslie Hamilton Gearren.
 * Gearren is also used in a Deleted Scene that appears to show Schwarzenegger in a mirror while Hamilton is working on the inside of his head; this was actually a clever set piece done entirely without camera effects through the use of an empty mirror frame, the Hamilton twins, and a Schwarzenegger dummy with an open robotic skull. They also use Leslie in Sarah's dream sequence.
 * The twins Don and Dan Stanton allow a mental hospital security guard and the T-1000 in his form to appear together. Those two also played two scientist twins in Gremlins 2. They also had a minor part (a cameo, basically) in Good Morning Vietnam.
 * X-Men: Amusingly enough averted in the first movie. Mystique impersonates Bobby "Iceman" Drake (Shawn Ashmore), and in one scene, the real Iceman walks by a door shortly before the fake Iceman comes out of it. The director states in the commentary that he didn't know he'd managed to hire somebody with an identical twin who could easily have played the double and went to the bother of doing two takes spliced together. They did make use of twins in the sequel though, when Mystique walked past the janitor while doing some undercover work.
 * And the favor was returned on Smallville, where Aaron Ashmore plays Jimmy Olsen, and Shawn Ashmore makes the occasional double work possible.
 * These same twins were also used for the role of Jake in the Animorphs TV show, listed below.
 * Funnily enough, Aaron Ashmore (Jimmy Olsen) had the part of BOTH twins in a movie called My Brothers Keeper, while they used Shawn for stuntwork.
 * Penélope Cruz's younger sister Mónica (who looks exactly like her) was used as her double in Pirates of the Caribbean: On Stranger Tides to hide the fact that Penelope was pregnant.

Live Action TV

 * Buffy the Vampire Slayer
 * In "The Replacement", Xander is split into the strongest and weakest elements of his personality. Nicholas Brendon's twin Kelly Donovan guest starred—but was only included when they had to be in the same shot together, due to the fact that Kelly Donovan isn't (primarily) an actor.
 * Also, Nicholas Brendan was so sick in "Intervention" that Kelly played Xander for his fight scene. He went on to be Nicholas's stand-in throughout the rest of the series.
 * Dollhouse: in "The Attic" Enver Gjokaj's non-actor twin was used a dream where Victor had to continually fight an enemy version of himself.
 * Sliders: For three episodes, Rembrandt's double was played by Cleavant Derricks' twin Clinton Derricks-Carroll. In the first of these, they actually looked so different that it was preposterous Rembrandt's friends could get them confused. (The show even included a cringeworthy "how'd they do that?" scene with the doubles repeatedly circling each other in awe and touching each other's faces.) In the second, makeup made them sufficiently indistinguishable that they traded roles for the final scene, with Clinton playing the Rembrandt who slides away with the group. Few viewers noticed. In the low-budget Sci-Fi Channel seasons, Cleavant played all the Rembrandts.
 * Full House: During eight years of alternating in the role, put Mary Kate and Ashley Olsen on screen simultaneously several times: a nightmare scene where Uncle Jesse sees two Michelles, when an identical distant cousin visited from Greece, a Good Angel, Bad Angel scenario, and in a dream sequence in the final episode when Michelle had amnesia, and the other Michelle represented the memories that she'd forgotten. Also made use of in the standard child labor law workaround.
 * 17 Again: The entire plot is that the grandmother is rascalized... and becomes the spitting image of her granddaughter, played by Tia Mowry. The rascalized grandma, naturally, is then played by Tamera Mowry, Tia's twin sister. (And just for kicks, the granddaughter's younger brother is played by...the twins' younger brother Tahj.)
 * Big Brother: Reality TV example: In the Australian Big Brother house, a pair of twins posed as one housemate and would swap in and out at Big Bro's command. This happened in a season of the American version as well
 * Animorphs: Jake gets taken over by a Yeerk. The team needs a fake Jake while he recovers from the infestation. Ax morphs into Jake, a role played by the actor's twin.
 * Juken Sentai Gekiranger had an episode where the MOTD creates an Evil Twin of GekiBlue, who was played by the actor's identical twin brother. These two have a habit of showing up in each other's shows.
 * In episode 25 of Gosei Sentai Dairanger, when the Dairangers fought evil clones of themselves, Kazu's evil clone was played by Daisuke Tsuchiya, the real-life twin brother of Kazu's actor, Keisuke Tsuchiya. The roles were later inverted in B-Fighter, where Daisuke was the main hero (Blue Beet), while Keisuke played his evil twin (Black Beet).
 * When Jill Hennessey of Law and Order was playing Claire Kincaid on a crossover with Homicide: Life on the Street, she was mostly absent from the L&O set in New York. The producers brought her identical twin sister Jacqueline—an occasional actress—in to play Claire in the episode "Corpus Delicti" in wide shots, reaction shots and inserts, while Jill still filmed the dialogue scenes.
 * In 1977, a year after Deirdre Hall began playing Dr Marlena Evans on Days of Our Lives, her twin sister Andrea was brought in to play Marlena's twin Samantha. Samantha was later killed off, but Andrea Hall again returned to the show to play Marlena's doppelganger Hattie from 2000-2001.
 * Mexican telenovela La Usurpadora used actress Gabriela Spanic's twin sister Daniela for a fight scene and a few other scenes that would have been tricky to film with one actress alone.
 * The Sid and Marty Krofft show Electra-Woman and Dyna-Girl did an episode where the evil Spider Lady turns herself into a look-alike of Electra-Woman. Once again, Deirdre Hall was Electra-Woman, while the Spider Lady doppelganger was played by Andrea Hall.
 * Scoundrels inverted, Patrick Flueger plays both Cal and Logan West. Coincidentally, they don't appear in many scenes together.
 * An episode of The Latest Buzz involves Amanda dating a guy who looks exactly like Wilder, naturally he was played by Munro Chamber's twin.

Professional Wrestling

 * While not actually a twin, wrestler Kurt Angle's older brother Eric looks quite a lot like him and is also a wrestler. On several occasions, WWE pulled a "switched Angles" plotline to let Kurt cheat his way to victory while his opponent was cluelessly attacking Eric.
 * Also there's the Bella Twins, Brie and Nikki. Brie debuted first and, whenever she was in trouble, she rolled under the ring to let Nikki finish the match. Now they wrestle as a tag team.
 * Possibly subverted during a match with Maryse: Brie was hurt, so Nikki pulled her sister out of the ring while the ref was busy chastising Maryse. Maryse saw the switch and nailed Nikki with a French Kiss.
 * Parodied in a match where the twins wrestled Maryse and Jillian Hall. Jillian rolled Maryse out of the ring and lay down in her place attempting to pull the switch. Of course Jillian and Maryse aren't twins and the ref wasn't fooled.
 * When the WWE aquired Dave Hebner's twin brother Earl from NWA, the creative got an idea where Ted DiBiase would hire Earl to impersonate Dave in order to screw Hulk Hogan out of the title. Thus, the infamous Evil Twin Referee match was born.
 * Another not actually a twin example, the wrestling team "The Killer Bees" often donned their masks at the end of a match so that they could swap without using a tag and not have the referee realise the wrong man was in the ring.

Western Animation

 * Semi-example; when The Who made a guest appearance as themselves on The Simpsons, Pete Townshend had a sore throat and wasn't able to read his lines, so his soundalike younger brother, Simon Townshend, read the lines on his behalf.

Real Life

 * Former professional baseball player and known goofball Jose Canseco once tried to have his twin brother Ozzie take his place at a celebrity boxing match.
 * Mario Andretti qualifies in second place for the 1969 Indy 500, then while practicing crashes and suffers facial burns (fortunately, much more common then than now). His injuries don't affect his plans to race, but they do affect the traditional front row photo, where the top three qualifiers pose in their cars. No problem, just get twin brother Aldo to pose in your place. Fellow front-row qualifiers A.J. Foyt and Bobby Unser don't even notice the stand-in.
 * Without spoiling the actual illusions, let's just say that stage magicians employ a lot of twins for their "lovely assistants."