The shutter in the Leica SM (screw mount) cameras has a little known feature that most owners are unaware of. That is its ability to take double exposure with perfect registration, not only on the last exposed frame, but any frame from the first one to the last exposed one.
The procedure is quite simple. Once the last exposure is made, turn the rewind lever to “R” and rewind the film slowly. While doing so, watch the rotating shutter release button. It will make one full revolution for each frame on the film. Rewind the film a little bit past one full revolution, about ¼ turn, and then set the rewind lever back to “A” and turn the film advance knob until it stops and take the second exposure. It will have perfect registration.
This procedure will work not just with the last exposed frame, but any frame prior to it. Use the same procedure, but count the number of revolutions of the shutter release, depending on how many frames you wish to go back. For instance, if you want to make a double exposure on a frame 12 frames back, rewind the film for 12 revolutions of the shutter release button, go a bit beyond that (1/4 revolution), set the rewind lever back to “A”, advance the film until it stops and take the exposure. The registration will be perfect.
It is advisable to practice this with an old piece of unexposed film. To do so, take off the lens and set the shutter speed to “B”. Then take a number of “exposures” and hold the shutter open to use a pen to mark each frame by outlining it in the open shutter. It also helps to number the frames consecutively. This allows you to practice the described double exposure procedure. Needless to say, repeating this procedure will allow for triple etc. exposures just the same.
To go back to exposing the rest of the film, put a lens cap onto the lens or secure it in some other manner to avoid any light from entering and advance the film for the number of frames you went back and continue shooting.
It is interesting to note that the Nikon rangefinder cameras as well as the first Nikon SLR model, the Nikon F, have the same feature since Nikon copied the Leica shutter for use in those cameras.