When You're Visited By A Copy Of Yourself, Stay Calm

You know Carl Linnaeus, right? The great Swedish naturalist who categorized plants and animals in the 1750s? He was a singular figure in botany. But when he got a headache, he stopped being singular. He doubled, from one Carl to two.

...the phantom might sit in Linnaeus' seat at his library desk, and Real Linnaeus, would, presumably, ignore him. One time, Professor Linnaeus was lecturing at his university and decided to run down to his office to fetch a specimen to show the class, and Critchley says, he got to his office, "He opened the door rapidly, intending to enter, but pulled up at once saying, 'Oh! I'm there already.' "

What a strange condition — to be visited by your perfect double, your doppelganger. Dr. Oliver Sacks, in his new book on hallucinations, calls these episodes "autoscopic doubles," and he cites a number of cases from medical history.