Could it be that the lensboard is not fitted properly and lets light in?
I know it's highly unlikely but...
Usually, lenses don't bounce light in odd ways like that. But since the lens is starting to separate, it's not entirely impossible.
One way to find out if it's the lens, if the lens board can be fitted upside down, try taking a single shot in similar conditions. If it's a lens defect, it should make the ghost image at an opposite place.