A dark double of yourself is read as the traits you deny or hide. In the tradition of depth psychology, meeting the shadow is the start of becoming whole.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.