i'm no expert on this but i have seen this happen in an 1100 semi auto shotgun.when the first shell is fired it forces the breach bolt open,when the breach slams back into fireing position its touching off another round.check the breach bolt to make sure its clean and allowing the fireing pin to seat back into place.maybe some grit keeping it sticking out some.like i said though i'm no expert and it may be more technical.