Basically, you have a pair of Always Identical Twins, or even Identical Strangers. Either they always dress alike, or they're fond of pulling the old Twin Switch... either way, nobody's entirely sure who's who at any given time. Except for this guy, who - SOMEHOW - can always tell 'em apart. Generally, it's not due to some sort of Sherlock Scan, but rather an emotional connection to one or both of the twins... See also Invisible Subtle Difference Anime and Manga
- In Ouran High School Host Club, the identical twins, Hikaru and Kaoru, delight in confusing people as to who's who, often turning it into a game (with varying degrees of seriousness). They're usually only distinguished by the way they comb their hair, and they switch that out at random. Even their own mother couldn't tell them apart, and indeed, NOBODY had been able to do so reliably - until Haruhi comes along and proves an inexplicable ability to tell them apart even when they're actively trying to be each other. In her own words, she's able to tell because Hikaru is one degree meaner than Kaoru...
- In Turn A Gundam, Identical Stranger Queen Diane and Kiel Heim switch places enough times to confuse everyone, imitating each other to such a degree that people targeting Queen Diane basically just shrug and decide that one's as good as the other. However, Harry Ord, the Captain of the Queensguard, becomes able to tell them apart after a short time. Not because of his dedication to the Queen (though he's certainly got that), but because he falls in love with Kiel Heim. When asked, he claims to be able to tell due to them displaying 'different kinds of sadness'.
- Brock in Pokémon once explained the differences between the Officer Jennies that were providing security for the Indigo League.
- The Donald Duck comic An Eye for Detail by Don Rosa is completely based on Donald's ability to tell Huey, Dewey and Louie apart.
- In Serenity, Fanty and Mingo in the bar seem to like to pretend to be each other, but Malcan apparently tell the difference.
Mingo: How is it you always know?
Mal: Fanty's prettier.
- Beltira and Belkira in the The Belgariad don't (seem to) try to confuse people, but when identical twins dress alike it happens. Polgara can always tell them apart, while the rest of Aldur's disciples have trouble.
- In Hellspark by Janet Kagan, Om im tells an anecdote about how he used to be friends with a pair of identical twins and was the only person who could tell them apart. The twins themselves don't appear in the story, but the method by which he told them apart becomes an important plot point.
- In Robert A. Heinlein's The Number of the Beast, Hilda Burroughs is the only non-family member who can tell identical clones Lapis Lazuli and Lorelei Lee apart.
- Subverted in Harry Potter, with Fred and George who are described as identical yet everyone can tell them apart.
- In one Baby Sitters Club book featuring a pair of twins, their mother can always tell them apart even when they are dressed identically. Mallory eventually learns how to do it as well.
- In Atonement, there are Briony's cousins who are identical twins. Reportedly, only their father can tell them apart, based on their freckles.
- In The Inn at the Stony Table, the gorgeous twin sisters in their early twenties are unrecognizable. Their father says he can only distinguish them according to their habits — as a toddler, one of them was ashamed to go potty in front of other people, while now one of them hates dill sauce. Their suitors (who are also brothers) are the only people who can tell them apart. This is when their father realizes the matches are worthy to be pursued.
- On Happy Days when Potsie & Ralph start rooming together they were going to go out with a pair of unseen twins who live upstairs. Potsie insists that he wants to date Maisy instead of Daisy. Ralph says it doesn't matter since they're identical, but Potsie insists that Maisy has whiter teeth.
- On the The Patty Duke Show, Patty's brother Ross could always tell identical cousins Patty and Cathy apart.
- Kaamelott: When asked by one of his mistresses how he can tell two twins apart (both of whom are also his mistresses) Arthur can't put it into words (possibly because the twins are there, and his male relationship survival instincts kicked in).
- Suikoden V - though Frey and his body double aren't related, the two can easily pass for twins while Roy is disguised as the prince. So much so, that he can even fool Frey's closest friends (including Miakis!) Despite this, Lyon is the only one who can tell them apart; whereas the prince's own aunt, Sialeeds, cannot.
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