• Open non-local container (inventory in Vehicle/Weapon Box). If the container is local to you, ask another player to sit as driver /take it in hands/. The container should have a weapon in it. If it doesn't, put your weapon in it.
• Right-click a weapon (as you would to put it in a weapon slot in your inventory) and then very quickly left-click it and drag it to the ground.
• Observe that you have this weapon in your inventory and a duplicate of it on the ground. This can be repeated ridiculously multiple times.

I am a regular player of Sa-Matra's Wasteland mission and it is really upsetting to see how the gameplay is being ruined as the duping method becomes more and more widespread.