Derived from the German words Doppel (meaning double) and Gänger (for goer), a doppelgänger is thought to be your body double — but not someone you'd look forward to meeting!
More an evil twin than a congenial clone, doppelgängers bring trouble when they show up as a ghostlike glimpse out of the corner of your eye. Some say they warn people in your close circle of family and friends that you're in danger of injury or illness. Seeing your own separate self in bilocation, or being in two places at the same time, may be your first clue to imminent death.
Doppelgängers pop up in literary fiction, legendary sightings, and even scientific study. Edgar Allan Poe centers his short story, William Wilson, on the vexation of the protagonist's haunting duplicate. Abraham Lincoln was reported to have seen two likenesses of his own image in a mirror shortly after his election to the presidency in 1860. Known for his superstitious nature (and a few eyesight issues), Lincoln tried hard to re-create the apparition for others to witness. The doppelgänger didn't return.
In a university setting in 2006, medical professionals produced sensations of a doppelgänger presence during the course of a patient's treatment for epilepsy. The discovery yielded insight into how certain areas of the brain respond to stimulation, and how some out-of-body illusions may be linked to psychological disorders.