If the same spot is being overexposed on both negatives then you are likely to have a problem apart from the holder. If the orientation is opposite each other (from side to side) then the holder may be involved. Does it occur on the "notch" end of the film? Or is it at the light trap area? I thought that I had problems with holders as well a while back. I found that I had pin holes in the bellows instead. I found this by loading a holder into the back, removing the lens board and holding a lit flashlight in the camera in a darkened room. Good luck.