I had the same experience one time last year while shooting clays. It has only happened that one time, but it sure scared me and everyone else in my group. I had just shot both rounds, and broke the gun open and reloaded. Soon as I snapped it shut, one barrel fired.
Just prior to this outing, I had the gun inspected, repaired, cleaned, glass bedded, etc... So I was quite surprised to say the least. Goes to show that one MUST always have the muzzle pointed in a safe place, no matter what.
Well after that mishap, and the few minutes afterwards that it took to remove the debris from my shorts, I looked things over to see if I could find a problem. I only saw one thing, but not sure if it could cause the problem or not. I was shooting Federal Vintage Gold shells, 7/8 oz loads with 5000 psi from Midway. Those shells burn very dirty. One shot and your barrel is littered with large flakes of residue. There were a few large flakes of this debris that filtered down and smashed on the breech face. Could it be possible that one of these flakes got sandwiched between the primer and when I closed the action, it caused the primer to fire?