Psychological Doubling
Pairing the main character with a second character who mirrors them. The double shows what the hero wants, fears or secretly is.
8½
Guido's wife Luisa and his mistress Carla are romantic rivals, and they are also 2 sides of his divided self. Luisa is what he respects and can't live up to. Carla is what he desires and can't respect. Neither is complete. Together, they make up the impossible ideal woman Guido keeps trying to make films about.
The Tenant
Trelkovsky starts to become Simone Choule, the woman who lived in the flat before him. He orders her usual drink at the cafe, smokes her brand and puts on a wig and her dress. By the end he repeats her fall, so the double is what he fears he is turning into.

