Soldato
Double exposure on one piece of photographic paper, or even the negative. It wouldn't be difficult to do at all.
Accidental double exposure
As I said, double exposure has apparently been ruled out as an explanation.
I wouldn't say it was a "fake" since that sort of implies that the person was trying to make it look like there was a ghost there. I'd say that looking at the year that photograph was taken and considering that personal photography was still a relatively new thing in those days and the technology was still in it's infancy I'd put it down to a fault with the camera/user.