I was going to ask what Robert did - what lens are you using? I'm thinking the lens might have ghosts that for some reason become more apparent when the reciprocity fails.
I'm having a difficult time believing that the film is sliding in the holder almost exactly the same way every time.
Chris:
Film movement in the holder is actually a very common problem, especially when you are working with long exposures. Anyone who has very much studio experience working with the long exposure times dictated when working with hot lights is well aware of the problem. Those who are knowledgeable are well aware that it is almost mandatory to tap the film holder lightly in the palm of your hand before inserting it into the camera in order to make sure that the film has fallen to the bottom (which side of the holder that is the bottom depends on whether one is shooting a horizontal or vertical composition). Doing so will prevent the gravity-induced movement.
If you will hold the film holder in a horizontal plane parallel to the ground, you can hear the film move around inside the holder if you quickly move it back and forth from side to side.
Robert Mapplethorpe has a "famous" portrait of a princess from somewhere in Europe that has the double image you will get if the film moves during exposure. If your double image is similar to one he got, then you problem is definitely film movement in the holder during a long exposure.
I would tend to blame your lens. If you shot 16 sheets and EVERY SINGLE ONE had it, I think the "popping film" is less likely. Borrow a second lens and next expedition shoot the same picture with each lens - - mount your lens, shoot picture, mount second lens, shoot picture. Does this happen when you are shooting into a light source? That has happened to me -- a lit lamp image was double -- perhaps reflections from wet plants, etc., are causing the problem.
Are you using filters and shooting into a light source or bright reflection? That, too, caused a double image once in a picture a friend took -- all the film he shot with the filter in place had a double image, the film he shot without the filter did not. Go figure. Get rid of the filter or try mounting it behind the lens.
I can't think of a lens-based problem which will get stronger merely because the light is weak. I suppose the back of your aperture blades could be shiney with wear, but you'd see that straight away.
Try mounting the lens with the lensboard rotated through 90?. If the ghost image doesn't rotate it's highly unlikely that your lens is at fault. If it does rotate, bulging film can't be the issue.
One last option: your lensboard is loose, or the lens is loosely mounted to it, or the front standard wobbles. Think in terms of a wobbly table with one leg too long so that there are two stable positions. The double image arises when the lens rocks or moves during the exposure under the (random) influence of vibrations or wind.
Chris,
There is one more cause for your double images that I have experienced that you should probably check out. A tripod leg that is slipping slightly during the exposure. I have experienced this in two ways. One was in soft sand, and another occurred when the plastic sleeves the joints of my Gitzo tripods became so wore that they began to slip.
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