I discovered image doubling while photographing my coppers. The image showed doubling, but when I looked at the coin, there was none. Lighting plays a role I believe and reflectivity on better coins than I have probably produces the same image doubling. It might also have something to do with the fancy electronic enhancements in almost all modern digital cameras.
over-sharpening a photo will sometimes show as doubling. other times the angles of the light do the same thing and a film camera would pick up the same thing. be careful from conclusions based on limited photographs.