Years ago I figured out how to get double exposures. Great for "Virgin Mary" appearances, etc. Works for sure on Alpha-1's.

1. Take any old sx-70 picture you do not want and cut just the picture out with a razor blade, leaving the bottom and white edges intact.

2. Keep the black cardboard cover thet is ejected when loading a new film pack.

3. After taking at least one picture from a fresh pack (you need the room in the film pack), take the camera and the black cardboard into a totally dark room.

4. In the dark room:
- remove the film pack,
- carefully depress the film pack down and insert the previously cut out picture,
- carefully insert the black cardboard cover and reinsert the film pack,
- the black cardboard will eject. You can leave the dark room now.

5. Take the first exposure. The cutout picture will be ejected. The next picture will have been exposed and still be in the film pack.

6. Take the second exposure. The camera will eject the double-exposed picture.

I don't know if this works with other film packs, but is probably worth a try.