The topic of Double or Multiple Exposures has always intrigued me. Sometimes, it happens quite by mistake.
Back in the film day, it was a common practice to change film type in mid roll, which meant that you took a roll of film out of the camera whether it was done or not, and changed to a different type of camera. This resulted in numerous accidental double exposures. Even though I would write on the film leader, sometimes when reloading the partial roll, there would be overlap. And, occasionally, an entire roll would be double shot. Usually, the film would be just ruined...but occasionally, you would end up with some pretty neat exposures.
Take for example this first image. I had been shooting on the roll of film...and shot a pictures of a students, Mark Beall, of Central HS, Beaumont, Tx. At some point, Mark, grabbed the same roll of film out of my bag and did not notice that the film had already been pre-exposed...and proceeded to shoot pictures of a football game.
So, the resulting image is OF Mark Beall...with a background image that he took.
Intentionally creating a double exposure use to require a certain camera...one that could be tricked to NOT advance the film. The Pentax K1000 was one such camera. It was (and still is) possible to keep the film from advancing while the shutter was re-engaged allowing you to take multiple pictures on the same negative.
Of course, there is also double or multiple printing which is done in the darkroom, as well as multiple flash resulting in multiple images on the same negative.
With the advent of digital camera, creating double exposures was left to the computer level of post processing and creating a Photoshop creation. IN the last couple of years, In Camera double exposure was introduced by camera manufactures to once again simply the process of taking multiple exposures.