William Shakespeare had great fun with them, sexual fantasies are full of them and they're at the heart of many a mid-day TV thriller, but what's it actually like being a twin?
Tonight's ITV documentary will probably be as close as many of us get to experiencing life with a mirror image, with several sets of twins describing it in a series of moving, thrilling, sometimes chilling, anecdotes of their similarities.
Anais Bordier and twin sister Samantha were brought up thousands of miles apart, but found each other by chance
More surprising are the lists of differences, for example, how twins might react to the same illness, and proof of this is in a sniffer dog being able to suss out who's who in an impressive five seconds. But all these scientific disparities aren't as much fun as the likenesses between identical siblings - no wonder they remain objects of such fascination for us non-twins, or "half-people" as twins apparently refer to us.
Some of the stories in this programme are profound, including the history of Anais and Samantha, born in South Korea, but adopted and brought up thousands of miles apart. When they were adults, Anais moved to London, Samantha to Los Angeles, and they discovered each other by freak chance - when a friend told Anais he'd seen her on an online video. When the pair were finally reunited at the age of 26, it was like coming across a missing part of themselves.
Samantha recalls: “I think we spent five minutes looking at our teeth, and our nose and our eyebrows and hands.” Anais adds: “Getting our DNA results was the last stitch in the pillow. Knowing that everything we’d hoped for, and already knew, was actually true.”
One in four sets of twins are born "mirrored" - ie everything's the other way round on one of them
One in four sets of twins are born, physically mirrored after their embryos split later than normal in the womb. Christie and Louise are one such pair, which means Christie is right-handed, Louise left-handed. But their minds are exceptionally in tune.
The sisters say: “We always agree don’t we? We’ve got the same ideas. We often have the same dreams. We always have the same thoughts. It’s like talking to the same person. It’s like we’re one brain.”
No wonder the Bard got so much mileage out of these special people.
'Secret Life of Twins' is on tonight at 9pm. More pictures of twins below... can you spot the difference?