In film photography a double exposure or multi exposure is where intentionally or unintentionally multiple images are captured on the same piece of film. This usually works best with a lower light sensitive film thus allowing more exposures.

Anyhow this is a little different, this is an overlapping exposure, where the film was only advanced part way, to create an overlapping image effect. Here the original image was taken to ensure there would be some darkness to the lower left, then a guessed advance of the film made (with the winder disengaged for some of it), then I lined up an image that would hopefully expose again in the darker area of the first image.