This is a common issue I hear about with the stoeger SXS's. I even experienced it with a brand new one out of the box. after a lot of arguing with Stoeger and sending one in for repair they still tell me it is caused by "bounce back" The way they tell it is that on recoil your finger moves forward enough to reset the trigger but still has enough pressure on it to fire the second barrel. I did find that if I do not hold the gun real tight to the shoulder it will double just about every time but if I tighten up my grip and pull it tighter into my shoulder it is fine every time. IMO they have an issue with their trigger assemblies and a loose hold is their excuse for not correcting it.
CPL holder since 1991, NRA Lifetime Member, DU Member, NICET certified
"For those who would give up essential liberty to purchase a little temporary safety, deserve neither Liberty nor safety.—Ben Franklin.”