Did the gun make a "boom-boom" or just a "BOOM" when it doubled? If it is the first, it is probably the intercepter causing problems (assuming a single trigger) and if it the latter, it is probably a worn sear causing the second barrel to go off with the first shot impact. My guess is that it is the first type which can be intermittent, dictated in part by how you shoulder the gun.
I agree that this is work for an experienced gunsmith and not a "home job".
Good luck
Z