I had one old J Stevens 325 double fire upon closing once. I never figured out the cause for sure & could never get it to do it again. I really treated it pretty rough (Unloaded) to see if I could get a repeat & it never dropped the hammers again. It was timed to the point you could load it without it cocking, so all I ever came up with was that the sear didn't fully engage, but just caught enough to hold till it closed. The gun wasn't slammed shut either, just closed normally. After that I always made certain I opened it far enough to be certain of full sear engagement & it never occured again.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra