Ammo Duplication is a glitch used to duplicate weapons dropped by a killed enemy. Using this glitch, you are able to get potentially infinite amounts of ammunition.
When you kill an enemy (e.g. grunt), they drop their weapon which you can pick up. If you kill them and save the game before their death animation has finished playing, reloading the save will cause the dying animation to restart, making them drop their weapon once again.
You can loop the process to generate infinite amounts of ammunition. This is especially useful when an enemy drops an SMG grenade or any other explosive weapon since you can use them for Damage Boosting later.