I believe Stanley Kubrick got the idea to put Doppelgangers in his movie from Stephen King. In the novel on page 149 (Chapter 25) as Danny walks into room 217 and sees his reflection in the mirror , “He watched his double nod slowly,” and “Yes, that's where it was, whatever it was. In there. In the bathroom. His double walked forward, as if to escape the glass.” And this quote from page 37 (Chapter 6) of the novel as Wendy talks to Jack, “She had expected to discover his anger…. It was almost as though the Jack she had lived with for six years had never come back last night — as if he had been replaced by some unearthly Doppelganger that she would never know or be quite sure of.” In the movie Stanley Kubrick makes this happen. In fact Stanley Kubrick’s “The Shining” may actually be the artistic Doppelganger of Stephen King’s “The Shining”?