My Minolta 9xi allows for in-camera double (or triple, quadruple, etc.) exposure but digitals don't have that option because it's something much better done in software post-processing. I remember when I was 13 or something and I learned how to do that on my (parents') all-manual SLR. You had to unlock the rewind lacet while reloading the film so that it wouldn't really reload but the shutter would be ready for another shot. Pretty much all non-electronic film SLRs can do that. Another example of "just because on film you can do it doesn't mean that in digital it's cheating"