Doubling ?

[Fast Ed]

I have a 1838 S/42 that appears to be in excellent condition. All matching numbers etc. I have 2 mags. neither match the gun's serial #. I was at the range the other day and gave it to a friend to shoot. He experienced what at first appeared to be a live round stovepipe that ejected with the spent round. I had previously experienced such stove piping of a live round with one paticular magizine. I gave him the other magizine and this time it shot 2 rounds at once! For sure one went down the barrel but the other one shot forward --upward at a 45 degree angle. We confirmed the double with round counts as well as debris from the ceiling was on the floor abour 8 feet in front of the shooting booth. Obviously the gun was immediately put up and is now locked on my safe. What causes this to occur and what can I do to fix it for sure?