Double Exposure Effect In Photoshop

Double exposure is a photographic technique that combines two or more exposures onto a single image to create a special effect:
Definition
A double exposure is a combination of two or more exposures on the same piece of film or light-sensitive paper.
Effect
Double exposure can create a surreal feeling, and the two images can work together to convey deep meaning or symbolism.
Results
Double exposure can produce ghost images and mirror images that tell a story in a unique way.
Technique
Double exposure can be done in-camera or in the darkroom.