Dopplegangers

Dopplegangers are our exact doubles. Sometimes they can appear as a normal person, but other times they appear as ghostly apparitions. So what are they?
They are said to represent bad luck and anyone who sees their own doppleganger could be in danger.
The ancient Egyptians believed that everyone has a spirit double. 
Abraham Lincoln was said to have seen his own double in a mirror, two separate reflections which was said to be a bad omen. He died shortly afterwards.
​Have you ever seen your ghostly twin?