Someone might have already mentioned this, if so I apologize. But the first time I ever heard of that term was in teh Infantry. When you conduct an ambush, you double-tap any enemy soldiers down in the kill zone just before you search them for anything of intelligence value.
This way you make sure that the enemy soldier you are about to search is in fact, "no more".
I did not hear about the double-tap in handgunning until I bought my first 9mm (Ruger P-85) and started shooting some "Action Pistol" at Fort Meade.