OK, here is the update. Kutter was correct in saying that hitting the receiver with a leather mallet would not duplicate actual firing. I took a closer look at all the usual suspects, triggers binding, or the right trigger hitting the left sear and anything to do with triggers, springs or sears.
nothing unusual. I examined the hammer closely and found a hairline fracture right at the notch that engages the sear. I welded the fracture and the doubling has been resolved.