They're More Common Than You Think
When it comes to calculating doppelganger odds, there are two situations to consider: The chance that your doppelganger exists somewhere out there, and the chance that you'll actually meet them. So far, no consensus has been reached; in 2015, a professor at the University College London told the Telegraph that the sheer amount of variables in play makes it impossible to truly calculate the odds. That same year, however, an assistant professor at Cornell University explained to Live Science that there's "only so much genetic diversity to go around," and strong resemblances are bound to show up eventually.
Then there's the BBC Future investigation published earlier this year, which concluded that while you probably don't have an exact replica down to the centimeter, finding someone with an extremely similar facial structure isn't unlikely. Let's hope your double isn't getting into trouble on the other side of the world.
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They're Believed To Herald Bad News
If you're not unsettled yet, don't worry; the folklore surrounding doppelgangers gets really creepy, really quickly. According to myth, doppelgangers were considered ill omens; if you saw your friend's doppelganger, it was a sign of impending illness or bad luck. At least it's not a death omen — that happens when you catch sight of your own doppelganger.
They Cast No Shadows
If the lookalike is a perfect double, how were you supposed to know when you were dealing with a doppelganger? According to folklore, there's a single difference: Doppelgangers cast no shadow and have no reflections in a mirror or water.
They're The Origins Of The "Evil Twin"
There's a reason folkloric doppelgangers herald bad luck: They're often the ones who bring it. Traditionally, doppelgangers give advice to the person they're shadowing, but it's often designed to be misleading or outright harmful. Sometimes, they're able to plant ideas in someone's mind, and if that doesn't work, the doppelganger might wreak havoc by pretending to be their double in front of their friends and family — your basic "evil twin" stuff.
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They Make For A Great Story
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Needless to say, folkloric doppelgangers are a common figure in stories. Edgar Allen Poe dealt with the theme in the short story "William Wilson," and doppelgangers are the focus of Fyodor Dostoyevsky's The Double. Today, they can be found in several creepypastas and urban legends; you can check out a list here. Finally, remember that malicious, shadow-less doppelgangers are simply a legend — not that it makes the real thing any less unnerving.