I just have a Stevens 612 that doubled (full auto SxS gun). It started doing it once in a while then it did it all of the tiem after about 300 rounds had gone through it. What was odd was that the 2nd firing pin was still cocked even though both barrels had fired in rapid succession.
I didn't try it but I assume that if I'd dropped it while it was loaded that it might have gone off.
I shipped 'er back to Savage and got a 512 returned to me. I'm not complaining, the 512 is a better gun and safer.