Only works on items that you can throw and then pick up again. Throw an item (melee weapons, throwing knives), then while the item is still in the air, hit select and 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. Does not work for guns (they can't be thrown), or bombs/molotov cocktails (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 attempting to do this the duplication will fail, because there is nothing for you to pick up.
Infinite Character Inventory
Fill your weapon inventory but leave one empty slot. Initiate a Trade with a friend and have them give you two Weapons. Now you can pick up, receive, and hold as many weapons as you want. Be sure to always have at least one more than the maximum number of weapons in your Character Inventory at all times.
Easy Legendary Weapons
Find a weapon box/container and stand next to it for at least 4 hours (Real time). Afterwards, open the container and a legendary is inside. This works anywhere and at any weapon box.