Double exposures were common with old film cameras. They occur when the photographer neglects to advance the film after exposing a frame and exposes another picture on top of it, resulting in ghosted images. In the case of this photo, it looks as though the film was only advanced half way. Although I've blurred the faces, it is obvious in the original photo that the boy on the bottom is the same boy further up, only in a slightly different pose. Although it's a ghosted image, it's not a ghost.
As film cameras became more sophisticated -- even inexpensive point-and-shoot models -- they had mechanisms that prevented double exposures. And with today's digital cameras, I don't think it's even possible to accidentally create a double exposure.
Double exposures were often used to hoax ghost photos. The trickery was done either in the camera or later in the film darkroom by combining multiple negatives. One of the most notorious practioners of this hoaxing was William Mumler, who in the 19th century created many such photos, sometimes with famous people as ghosts. In the photo on this page, you'll see one of his more famous multiple exposures depicting the widowed Mary Todd Lincoln and the "ghost" of President Abe.