1) Place the item you wish to duplicate into one of your player's containers.
2) Enter the vehicle and open your inventory.
3) Drag the item you wish to duplicate to one of your other player containers and close the inventory (doesn't matter if you hit esc key or press the X on the inventory menu.
4) Open the inventory again and drag the item you wish to duplicate to the vehicle's inventory, close the inventory and exit the vehicle.
5) The item should now be in the vehicle's inventory and in the player container you dragged it to.
Video displaying this exploit with a pistol and grenade: https://youtu.be/X6hLJBfUAPc