In photography a double exposure is the superimposition of two exposures to create a single image. In other words, you take two shots using the same segment of the film and as a result you have a blend of two images.
For this effect we've already chosen a set of images you can use as a texture. All you have to do is upload your photo and choose one of those textures. For better results your photo should be a high contrast portrait on a plain light background.