The act of accidentally taking 2 pictures when only meaning to take one. This is often followed by a ton of people becoming really pissed off because they dropped out of their poses and therefore the second picture looks terrible.
Person 1: Excuse me sir can you take a picture of us?
Person 2: Sure no problem. (Double-Snaps)
Person 1: Damnit I probably look like an idiot in the second one! Nice going ass hole. I asked for one picture and you end up Double-Snapping