Canon A-1 - Manual (Page 130)

You can make any number of exposures on
the same frame simply by repeating the
above procedure before turning the film
advance lever. There is a possibility of a
slight movement of the frame if you make
an excessive number of exposures on the
same frame, if you turn the film advance
lever too forcefully or if there is film slack.
The frame counter is stopped until you
actually advance the film to the next frame.
Once you have set the camera for a multiple
exposure, there is no way to cancel the
process before actually making it. If you
suddenly decide you don’t want a multiple
exposure but still want to preserve the one
or several shots you have already made on
the frame, you have the best chance of doing
that if you manually set the lens aperture
ring to the minimum aperture, the shutter
speed to 1/1000 sec., cover the lens with the
lens cap and then release the shutter.