Shelley's Frankenstein contains a protagonist, Victor Frankenstein, but that protagonist is not a "hero." Concerning doubles and identity, as you refer to this element, and Victor, the enotes Study Guide on the novel says this:
Victor Frankenstein’s creation has often been characterized as his “dark side,” similar to Robert Louis Stevenson’s Dr. Jekyll and Mr. Hyde, written almost 70 years later. The Frankenstein monster could be considered a model for Stevenson’s Mr. Hyde, the embodiment of all of Henry Jekyll’s less “proper” thoughts and actions. The monster is an untamed version of Victor’s ego, demanding and determined yet brutish and naïve. He expects things to go his way, and when they don’t, he responds with violence. The monster could even be called a manifestation of Victor’s “inner child.”
Victor’s split is reflected in the novel’s structure, which shifts from letters to journals to straight narration to what seems like intense first-person accounts. Victor is the narrator of record, but his observations are far too omniscient to be realistically his throughout the novel. Mary Shelley’s narrator actually shifts from perspective to perspective, using the best perspective and the best form (see “Language” above) to tell a particular part of the story.
The monster becomes Victor's "dark side," because Victor creates him, but he remains as he was born, instead of progressing into a fully developed human being, because Victor abandons him and refuses to nurture him. Victor seals his own fate, as well as the monster's, when he impulsively destroys what he has started of the female monster.
The most important double is Victor's double, the monster. The central issue of identity deals with Victor's different sides and the monster's lack of identity. And Victor is the protagonist, but he certainly is no hero.