The military art of placing two rounds in quick succession into same part of an enemy with the intention of causing him extreme discomfort.
This technique conserves ammunition (very important considering how tight the Treasury is and then theres poor supply from the MOD to be considered), ensures target falls when hit (with respect to poor 5.56mm stopping power), is armour defeating (puts 2 rounds within 2 inches of each other) and of course it will hurt like a mother fucker ... twice.
Although Spam special forces (Delta Force in 1961) claim to have invented this technique of controlled unpleasantness, it was infact taught to SOE personel 17 years earlier (1944). Certainly Brit and Aussie squaddies were double tapping in close quarters and jungle style fighting long before Vietnam kicked off.
A form of double tap was used during the Rhodesian Civil War were it was called 'Drake Shoot'. If attacked from ambush and the rebels were behind cover then the Drake Shoot technique would be employed. The first round fired (7.62mm) was intended to break up the cover (likely to be less than solid i.e. rotting tree trunk etc) and then the second follow up round would then kill the enemy.