I like to fiddle with cameras. The less invasive, the better!
A lot of cameras will do double exposures. Here is one way to find out if your camera will. The trick is simple and non invasive. After shooting the shot you want to double over: Hold down your rewind button while cocking the shutter. Watch the rewind spool closely while cocking, (it helps to make sure the film is tight), if the spool does not turn, you can make the double shot.
I used a Time Life type camera for this technique to show you how it is done.