I've had strange things happen myself from time to time.
First was with an SAFN-49 which actually WAS doubling.... and with mild handloads of fairly slow powder, at that: 2 rounds at a pop, 2 empties come out, 1 hole where I aimed it and another 'way high. Second primer, in each case, was DIMPLED rather than struck, so no fault with the trigger mechanism; it had to be a gas fault. Yeah, what I was doing with the mild loads and the slow powder was increasing dwell time at the gas takeoff. Simple solution: open the gas bleed so the piston isn't getting as much. Lays its brass on the bench now and no more screwing around. Nice!
Other thing was the first time I tried an AR-15. I was absolutely CERTAIN that the thing fired 2 rounds, but my friends only saw one empty come out, there was only 1 hole in the target.....and there was one up the pipe, 3 in the mag.... and I had loaded only a 5 count.
Analysed it and what I THOUGHT was a double-tap was in fact the effect of all that crap slamming around in the thing's guts; it FEELS like it's doubling. Only thing that PREVENTED it from doubling was the fact that it had a stock trigger: heavy, hard and a bit gritty. I think that if the trigger had been lightened and honed, the effect of the recoil alone could have been enough to MAKE it double: the rifle comes back into your shoulder, away from the trigger and the trigger resets..... then the rifle slams forward again when it chambers the new round..... and your finger is still in the same place..... and it trips the that nice, light, honed trigger.
If it were mine, I would be checking the firing-pin for binding and/or breakage (as mentioned previously) but I think I would also check the trigger mech. Ugly triggers can be a help some time and these things seem to like triggers that I wouldn't have on a real rifle made out of steel and wood (just the way James Paris Lee and Sir Charles Ross designed them!).
Hope this is of some help.