How 2 duplicate items:
You need 2 DS and 2 ACWW or a good friend on Wi-Fi.
First way: Person with items is hosting
Person opens gates, friend goes in. Takes items, leaves. The friend leaving resets DS when the arrow goes around 8 times. Both people should have items upon restarting game.
Second Way: Person without items hosting
Person opens gates. One with items comes in, drops items. One without picks up items and saves. One who originally brought items gets them back then leaves. One leaving resets DS when arrow goes around 8 times. Both people should have items upon restarting game.