This is very easy to reproduce. We mainly play the Breaking Point mod but it works in other mods as well. here are the steps:

1. get a buddy
2. place an item in a container (ground, tent, car, box, dead body, etc)
3. you and your buddy open the container.
4. both players place their mouse courser over the item
5. count to 3, and both players right click the item to move it to their inventory.

If you both click at the same time both will have the item in their inventory. You can do this with more than 2 people. We tested it with 4 people and all 4 got the item.

It took us maybe 3 trys to get the timing down. it has nothing to do with excessively clicking, or dragging the item around. It has to do with both players right clicking the item to move it into their inventory at the same time.

We were able to duplicate (pun intended) this, and if you have the timing down it is very easy and can be done by a group of people to duplicate an item multiple times at once. for example, a group of 6 can turn 1 magazine into 6 magazines with just a coordinated click of their mice. This is a show stopper in multi-player mods. Needs to be a top priority to fix.