Around 10 years ago I fired my Colt New Service. I never experienced 2 rounds going off in a revolver when only pulling the trigger once. What happened was the recoil plate(donut) had fallen out without my knowledge and I didn't notice it when I swung open the cylinder to load.
Apparently when that recoil plate is missing the gas blowing back when the primer ignites has enough force to send the hammer back again and then the hammer fell forward discharging the next round. At this time the firing pin embedded itself into the primer and it had to be freed in order to open the cylinder again. This scared the crap out of me.
I happened to have a recoil plate from a SAA that was slightly larger that had to be fitted. That little donut also had to be staked to prevent it from happening again. But other than that incident my 90 year old Colt never gave me a problem.