I had a similar experience last week though not a double exposure. I developed a stack of 4x5 holders and after all the sheets were hung and dry I discovered a barn on one sheet. I swear to God I did not shoot that barn. I shoot a lot of barns but not this one. In addition to regular spot metering with a Pentax spotmeter, I also always take a reference shot with the Pocket Meter app on the iPhone and sync that with Dropbox and I reviewed those photos and that barn was nowhere to be found. I quadruple reviewed my exposure notebook and nothing in there about this particular barn. I can only conclude it was a sheet already exposed (badly) in a sheet film holder I bought on "The Bay" but I clean out my holders with a brush religiously before loading new film so I should have caught that if it was in there. I like a good mystery but this is really weird.