Note: This must be done in solo mode. When you reach a checkpoint during any campaign quest, drop the item on the ground you want to duplicate (if you want, all your items in your inventory). Then, kill yourself by using deobombs or molotovs, When the respawn timer starts counting down, pause the game and reload your checkpoint. When you respawn, all the weapons you dropped will be back in your inventory and also still be laying on the ground where you dropped them. You also do not suffer the cash penalty from dying since you reloaded the checkpoint.
Note: This trick does not work with projectile weapons. Hold LB then hold RB + Y. If done correctly, there should be two of the same weapon on the ground and it will show that you threw and dropped the same weapon. Repeat this as many times as desired. You can use this trick to make money in game or in online play, and you can supply teammates with strong weapons. You can also duplicate a weapon instead of repairing it. Note: This also works with modified and upgraded weapons.