In multiplayer with a friend, drop all of your items and exit without saving. When you come back, you'll still have the items you dropped, and the second player can give you the items you dropped back and you'll have two of everything.
To get more items, when you play bosses that let you get an attack oppertunity, use it against them, they will always drop an item when you do it, and if you kill the boss with it you get two items. It only works on bosses.
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