In Real Life, identical twins are still often different enough to be distinguishable. In Fictionland, this isn't always the case. Many times a twin will be mistaken for their sibling, whether or not they realistically would be.
This may happen intentionally, as part of a Fake Twin Gambit, a Twin Switch, or if a character is Impersonating the Evil Twin, but just as often it's a complete accident, with hilarity following. However, this can also be used for drama, if the mistake leads to an actual problem rather than a gag; this route is common when one twin is a criminal. If the twins are babies, expect this to happen often.
Contrast Identical Twin ID Tag, where the twins make a conscious effort to avert this trope. Compare Ambiguous Clone Ending, where the characters and the audience is left unsure who the clone is. Also compare Identical Stranger and Inexplicably Identical Individuals, who may also be confused for each other. Supertrope to Twin Switch and Impersonating the Evil Twin, as those tropes rely on this confusion to work.
- In The Quintessential Quintuplets, when Fuutarou and the Nakanos all end up in class 3-1 together, it's shown in chapter 70 that their classmates are even worse at telling the quintuplets apart than Fuutarou is. This leads to situations like a pair of girls trying to bring one of the sisters to a teacher and grabbing the wrong one by mistake (three times in a row!), or other students asking Fuutarou to pass a message/item to a quint that's standing right next to him.
- In Zekkyou Gakkyuu, Nana and Nono from The Story About Twins get mistaken by sight alone. Despite them having different haircolors.
- Wonder Woman: The Heyday triplets (three of the Holliday Girls) are identical and are commonly mistaken for each other even when they're not pulling a Twin Switch, which leads to them being ordered to wear name tags. They generally avoid the name tags, which comes in handy when their vile elder brother abducts them to make them sign over their inheritances. It doesn't work because he can't tell which one should be signing which name.
- DC has a mother-daughter example. One instance of Green Arrow meeting Black Canary has him saying that he's had a huge crush her for years. Black Canary is offended because he's talking about her mother.
- Features in the Modern AU How to Train Your Dragon fic Confusion, as Astrid and her OC twin sister Lillian start dating Hiccup while involved with each other; initially all three were unaware that the sisters were dating the same person while Hiccup assumed that he was dating one girl who acted differently in clubs and on campus, only realising that there were two of them when he was invited to their apartment. After they settle into a triad relationship, as part of their efforts to stop their mother finding out that Hiccup is dating both girls, at one point Lillian pretends to be Astrid when their mother comes to the sisters' flat while Astrid is out (their parents having been told Hiccup is 'just' Astrid's boyfriend).
- In The Bridge, the Sirens and Dark Hunters get a bone to pick with Twilight Sparkle after she allies with the dark magic source the Hunters are going after along with stealing Sonata Dusk's necklace, leaving her scarred and sickly. This causes Monster X to threaten the resident Rainbow Dash, knowing it would draw the hiding Twilight out. So when Princess Twilight Sparkle arrives after being summoned by Sunset Shimmer, it's not even two hours into her arrival when Monster X and Gigan ambush the party and kidnap her to demand information and the return of the necklace. Problem? She has no idea what they are talking about. It's only after discussion they realize it was the human world's resident Twilight Sparkle that was responsible.
- The twins from the Firefly movie Serenity try to invoke this by pulling the Twin Switch on people for fun. They're frustrated that Mal can always tell which is which.
Mal: Mingo. [Nods to Mingo] Fanty. [Nods to Fanty]Mingo: He's Ming-Mal: He's Fanty, you're Mingo.Mingo: [Stares at Mal for a moment before reluctantly smiling] How is it you always know?
- Twin Sisters of Kyoto: Hideo meets Naeko when he accidentally gives her the obi (a kimono's sash) that he weaved for Chieko. (Hideo does not know about Naeko because Naeko and Chieko were Separated at Birth.)
- The Chronicles of Narnia novel The Horse and His Boy. After Prince Corin runs away from the Narnian group, they later meet Shasta and mistake him for the prince because the two look exactly alike. It's later discovered that Shasta's real name is Cor and he and Corin are identical twins.
- This sometimes happens with Fred and George Weasley from Harry Potter, even by their own mother. Often it's because the twins themselves pretend to be each other just to troll others.
- In Flawed, this is downplayed, occurring between sisters that look alike, but are months apart in age. Celestine, who was branded as "flawed", still has to go to her high school while marked, forced to always wear a red arm band. Her similar-looking older sister Juniper is one day photographed from behind, mistaken for Celestine, and since Juniper doesn't have a band, everyone thinks Celestine broke the rules. This causes Celestine to be expelled from school.
- In The Folk of the Air faeries repeatedly have issues telling Jude and Taryn apart. Locke, having wooed both of them, is so bad at differentiating the two that when Jude attempt to threaten him and tell him to treat Taryn well, he initially assumes Taryn has randomly become violent with him. Cardan even goes so far as to suggest that Locke couldn't tell which was which when he first tried to court Taryn, and it was chance that Taryn ended up being the first twin he approached. Taryn also takes advantage of how few people can tell them apart (as well as the fae rarely taking into consideration that humans are capable of lying) to claim to be Jude in order to take advantage of her status.
- The Twin Switch in Lottie and Lisa is so successful that the girls are only found out by complete coincidence. They look exactly alike except for their hairstyles which are easily disguised. However, they are also Polar Opposite Twins and have to work hard to maintain their charade, which causes especially Lotte a lot of stress and anxiety. They would probably have been found out due to their different personalities and lack of knowledge about each others' lives had it not been for the fact of the parents being distracted by their own problems. Their looks, however, are so identical that they themselves cannot distinguish between each other in a photo.
- In the first book of Sweet Valley High, bad girl Jess goes to a dive bar and gets involved in a fight. The cops mistake her for her more innocent twin sister, Liz, causing Liz to be the butt of rumors at school. This actually happens repeatedly throughout the series, to dangerous levels—Jessica's stalker kidnaps Elizabeth by mistake, while a guy stalking Elizabeth because she witnessed him committing a crime abducts Jessica.
- In a Sweet Dreams book, a girl runs into the guy she met a few days ago and happily chats with him, not understanding why he seems weirded out by her behavior. When she runs into him again and blasts him for his rudeness, he realizes that it's his twin she was talking to and that his attitude was because he thought she was coming on too strong (oddly, at no time did it occur to him that she'd simply mistaken him for his brother).
- Arrested Development:
- Happens repeatedly in the episode "The One Where Michael Leaves" with the police mistaking Oscar for his fugitive twin brother George. Except the last time when George deliberately disguises himself as Oscar to invoke this.
- Invoked again in "Righteous Brothers" when George gets Oscar sent to jail in his place. The following episode reveals that "You've got the wrong twin" is a very common retort to the police.
- Columbo once used this to trick a murderer into incriminating himself. The murderer had almost run over a blind man while fleeing from the scene of the crime. Later, Columbo brought in what appeared to be the same blind man to identify the murderer. The killer protested that the man was blind. Then Columbo asked why the killer would think that, since when the man he had brought in wasn't wearing dark glasses or carrying a white cane. The killer claimed that as a trained physician he could spot subtle clues. Then Columbo had the "witness" read from a newspaper. The "witness" was the blind man's identical twin bother, and by claiming the witness was blind the killer showed that he had mistaken the man for the blind man who was almost run over.
- The final moment of one episode of Criminal Minds reveals that they arrested the wrong person, with the closing quote being "There are only two things in life that are ever unexpected. Twins." The next episode clarifies that the unsub and his brother were separated as toddlers and even the unsub himself didn't know he had a twin brother. The arrested twin, Jesse, had insinuated himself into the investigation as part of a plan to make contact with the guilty twin, Wallace (and eventually Kill and Replace him). After the brothers have met, a coworker of Jesse sees Wallace leaving Jesse's house and assumes it's Jesse. He's too freaked out to correct her, so he just runs away.
- One episode of CSI deals with the Victim of the Week being a soccer mom that was shot in a parking lot, and the investigation eventually leading to a house where the woman's identical twin has apparently hanged herself. Turns out that the women were long-lost twins and the one who apparently committed suicide had become a newspaper editor and was about to rat out a photographer that had photoshopped pictures of the Second Gulf War to "make them more representative of War Is Hell", winning a prize for that. He had seduced her, drugged her and hung her, and seeing her twin made him believe that she had somehow survived that and shot the twin just to make sure.
- On Deception (2018) identical twins Cameron and Johnny Black have been using the Twin Switch as part of a magical act for most of their lives. Very few people know that Johnny exists which allows them to do spectacular teleportation tricks eg. Cameron disappears in an explosion on stage in Las Vegas and Johnny then appears on a similar stage in Times Square in New York City. In the series premiere Cameron is arrested for a crime committed in New York City and realizes that the police have mistaken him for Johnny. Once the twins' secret is revealed to the world, Johnny is sent to prison and Cameron tries to prove that he was framed. Once Cameron confronts the Mystery Woman behind the frameup ,she realizes that she framed the wrong brother. She once met Johnny but mistook him for Cameron and when she wanted to recruit him for her scheme, she framed the other twin to get him out of the way. She is quite annoyed when she realizes that she got the wrong brother sent to jail.
- Friends: As part of the shared universe with Mad About You, Lisa Kudrow plays both Phoebe and her twin sister, Ursula, who happens to be a character in the other show. This is joked about when the cast of Mad About You show up at Central Perk, and thinking Phoebe is actually Ursula, assume she's a waitress at the cafe (just as Ursula is a waitress at their local restaurant) and tell her their orders. Phoebe, naturally, is confused by this.
- House of Anubis: When Patricia's polar opposite twin Piper arrives at the house, this trope is in full force. Apart from their Twin Switch, Piper is mistaken for Patricia multiple times. This leads to Eddie kissing her, Alfie thinking his crush on Piper is a crush on Patricia, and the Sibunas thanking "Patricia" for getting rid of the stolen oxbow before the teachers checked her bag. Strangely, they did have an Identical Twin ID Tag with their necklaces. After Piper gives Patricia hers before leaving, Eddie then mistakes Patricia for Piper.
- iCarly: In the episode where Sam's polar opposite twin Melanie is introduced, Freddie spends the entire episode thinking that Melanie doesn't exist, and is just Sam screwing with him. As the two are never seen together until the very end, the episode is set up so that the viewers are just as uncertain as Freddie.
- A particularly heartbreaking version forms the basis of an episode of Law & Order: Special Victims Unit. A woman who had an identical twin threatened to report a crime syndicate to the cops because they were holding another sister hostage. A member of the syndicate went to her apartment to kill her, on the same night that it just so happened the other twin had come by looking for her sister. When the assassin found her there, he mistook her for his target and killed her.
- An episode of Nash Bridges involves a pair of twin brothers, one of whom is a city official while the other is a criminal. At one point, Bettina arrests the criminal, but lets him go when he claims to be the city official. Soon after, Bettina encounters the official and arrests him despite his protests. Being an idiot, Bettina doesn't ask for proof of identity either time.
- The first episode of Sister, Sister has Separated at Birth twins Tia and Tamara coincidentally at the same store. One is looking for a sweater and the other is looking for a sweatshirt. They take turns asking the same sales clerk, who doesn't realize there are two of them and slowly goes crazy with them for constantly "changing their minds." Considering they didn't even know they were twins yet, a complete stranger couldn't be expected to know either.
- The Suite Life of Zack and Cody: Downplayed. For the most part, the twins manage to avoid this trope, but one episode has them doubt their identities when they see a video of their mother getting them confused when they were newborn babies.
- In the Chilean radio program El Chacotero Sentimentalnote , there was a case of a guy who had a girlfriend and she has a twin sister. He tells about his relationship with his girlfriend but then sometimes he noticed his girl acted very different, to noticed time later she made Twin Switch with her sister, so he basically was dating both sisters without noticing it.
- In the second Boktai game, during the mission in the Remains, Django gets mistaken by Kid for his twin brother Sabata, who saved Kid earlier in the story. (How this could happen is confusing, considering that both twins have distinguishable hairstyles and hair and eye color).
- Before the events of Fire Emblem: The Binding Blade, Raigh stole one of Hugh's spellbooks. In a support conversation, Hugh mistakes Lugh, Raigh's identical twin brother, for Raigh. Hugh realizes his mistake and apologizes.
- Her Story: Suggests that the trigger for the murder was Hannah posing as Eve and Simon falling for the deception, giving her another "unique" mirror to show his love. At least that's her story.