A frame narrative, also called pericope or embedded narrative, represents duplicity in that it creates two threads of a story. For a illustrative random example, Phoebe Cary's poem "A Legend of the Northland" is a frame poem with narrative duplicity. The frame is a mother telling a story to her small son to teach him to be unselfish and avoid growing to be a selfish man. The embedded or framed story is about Saint Peter asking a selfish woman for something to eat because he has gone long without food. The woman ignores him and ignores him and in disgust at her selfishness, Saint Peter turns her into a woodpecker.
It is easy to see in this example that there are dual conflicts for dual character groups. The theme is the same, but the action, conflict, and resolution are not the same. In Dr. Jekyll and Mr. Hyde, the duality involves, first, the narrator and his life and story conflict and, second, the life and story conflict of Dr. Jekyll, which is tied as it is to his infamous alter ego Mr. Hyde.