Twins

Twins are genetically duplicate siblings. This is a real world phenomenon but fantastic cinema offers a number of variants on the theme.

The most frequent of these variants is the good twin/evil twin theme, which usually takes place in assorted psycho-thrillers involving masquerade and identity swapping/theft.

Another common theme is the belief that twins share a psychic connection or are able to intuitively communicate. Conjoined (or Siamese) twins feature in circus freakshow stories or as freakish villains.

For more detail and an overview of the genre see the Theme Essay Twins in Fantastic Cinema.