Shahar Arzy and colleagues of the University Hospital, Geneva, Switzerland, unexpectedly reproduced an effect strongly reminiscent of the Doppelganger phenomena via the electrical stimulation of a patients brain. They applied focal electrical stimulation to a patients left temporoparietal junction while she lay flat on a bed. The patient immediately felt the presence of another person in her "extrapersonal space". Other than epilepsy, for which the patient was being treated, she was psychologically fit.
http://en.wikipedia.org/wiki/Doppelganger