Generally when I rewind a roll after finishing it in the camera it will usually rewind so that the leader (the part of the film that sticks out so that you can feed it into the take up spindle) is totally gone. That would tell anyone that the roll is finished.

But if I didn't do it properly and the leader is still sticking out, I might stupidly just put it back into the pile of film to be shot and shoot it again. On my Polaroid 600SE, which never warns you if you had already taken the shot, it's extremely easy to make a double by mistake.

It's all okay. It's just fun. Some people purposely double-expose. I can admire the technique, trying to perfectly align all of the frames by rewinding exactly (otherwise you'll get lots of overlapping as I have here). And of course digital shooters will just do it in Photoshop. What's the fun in that?

Happy accidents is the name of the game.

Depending on the under-exposure of the first series, you might end up with some nice results. A lot of this roll is pretty crap, a big mess, but some of these accidental doubles seem to even have a theme.

The one with my coworkers at the top as we are um dialoging over alcohol has that interesting flower exploding from the right. It's as if my coworker John is espousing some powerful concept and the flower is the grace of it.

The vertical series (that I couldn't properly crop because the doubled exposure overlapped the frames), exposes a car over people walking on the street. Car vs walking. It's political!

Here's another one I did by accident with a Pentax K1000 a couple years ago that I like still.