If you shoot with a camera that doesn’t have a coupled film advance and shutter cocking lever, the chances are you have accidentally shot some multiple exposures by forgetting to wind the film on after each frame. This creates overlapped images and yields unexpected effects.
Results can be fairly unpredictable even when shooting multiple exposures deliberately, and cumulative exposure in overlapping areas will result in overexposure. If you aim to create one correctly exposed image from two exposures on the same frame in the same lighting conditions, you need to halve the exposure for a single correctly exposed frame.
Two basic examples:
Correct exposure at shutter speed 1/125 would need to be set to 1/250 for a double exposure (two exposures, each one stop underexposed), and 1/500 for a quadruple exposure (four exposures, each two stops underexposed).
Correct exposure at aperture f/8 would need to be changed to f/11 for a double exposure (two exposures, each one stop underexposed) and f/16 for a quadruple exposure (four exposures, each two stops underexposed).
The above assumes that you are shooting each exposure in the same lighting conditions and want equal emphasis on both exposures in the resulting image.
For deliberate multiple exposures with a camera that automatically cocks the shutter when the film is advanced, make the first exposure, then take up any film slack by turning the film rewind crank in the direction of the arrow until there is some resistance. In order to cock the shutter without advancing the film, depress the film rewind button on the bottom of the camera before cocking the film advance lever. This will cock the shutter without advancing the film so you can make another exposure on the same frame of film.
This can be repeated for as many exposures as you wish. When you are done with the frame of multiple exposures, advance the film and make one blank picture with the lens cap on (to prevent the multiple exposure overlapping with the next photo you take).