A doppelgänger is essentially an individuals "exact double." Theres are conspiracies saying that every single person in the world has a doppelgänger. Some even say that after you see your doppleganger in real life, you die.
Using the term doppelgänger in the paranormal sense, it would be a ghostly double of an individual who is still alive. There is belief that this is a ghost taking the appearance of an individual to cause mischief. Also, some believe that seeing the ghostly figure themselves can cause harm, illness, or death of the person.
It is said that President Abraham Lincoln saw his doppelgänger in the mirror next to his own reflection. The being looked sickly. Shortly after, the president was assassinated. Queen Elizabeth I was another famous person to have seen their doppelgänger. As she entered her bedroom, she saw herself laying in her own bed. The figure looked sickly. Shortly after, the Queen became ill and died in the very same bed.
The photos above are real life, alive doppelgängers. Below are ghostly computer generated doppelgängers. The bottom right is a portrayal of Abraham Lincoln.