Note: This works on items that you can throw and then pick up again. Throw an item (melee weapons, throwing knives) and while the item is still in the air, go to your inventory. Unequip the item that you threw, then leave the inventory menu and pick up the item. You should now have 2 identical copies of the item. As I mentioned, this does not work with guns since you can't throw them or bombs/molotov cocktails since they explode on contact and as such cannot be picked back up. Also, if you are using Logan and have his boomerang skill, if the skill procs while you are trying to do this the duplication will fail, because there is nothing for you to pick up.
At the Pinai Ferry Station there are twin zombies that must be killed. The first twin is at the beach to the north of the ferry station. It will appear from the lifeguard's shack on the beach. Once you kill it, go to the billboard with the pick-up truck on top of it at south section of the ferry station. Head to the back of the billboard and climb up the ladder. You can collect sniper rifle and bait from the truck. While up there, look toward the ocean and notice that the building to the left is open. Toss the bait near the doors to draw out the other twin.
Fill your weapon inventory until you have only one empty slot, then start a Trade with a friend and have them give you two Weapons. Now you can pick up, receive, and hold as many weapons as desired. Be sure to always have at least one more than the max number of weapons in your Character Inventory at all times.