Way 1: using 2 players (hard to reproduce every time)

1. Have player 1 drop an item on the ground (works in containers as well)
2. Sync the pick up of item with player 2 (I.E. count to 3 and both pick it up)
3. There will now 2 of the item (weapons sometimes lose attachments)

Way 2: using 2 players (easy to reproduce)

1. Have an item in a container or backpack.
2. Have player 1 left click and hold the item.
3. Have player 2 right click the item.
4. Have player on drag the item to the ground.
5. There should be one item on the ground and on in the inventory of player 2.

Way 3: Using 1 player (hard to reproduce)

1. Have an item in a container
2. Right click item and quickly left click the item and drag it to the ground.
3. There should be an item on the ground and inventory.