Minecraft wiki entry for Block_and_item_duplication, associated text: When an item frame is removed (broken by the player) from the rear of a piston as the piston is retracting, the correct timing will yield two items of the item that was within the frame. This will work with any blocks or items inside of the frame, with shulker boxes being able to duplicate multiple items at once. This method works in both singleplayer and multiplayer game modes.