Hmm, that is quite mind-boggling. I have two Grafmatics here and can't for the life of me come up with a mechanism that would make that happen.
Multiple exposures on one sheet could happen if you just forget to "cock" the next frame; you can open and close the dark slide however many times as you like. The overlap of the first and last frame is really strange. The only thing I can think right now is if you had only 5 holders inside instead of 6.