How do you duplicate in Pokemon Diamond and Pearl?

Answer 1

To perform this trick you must have a Pokemon that knows "Thief," and a Ditto. Enter a double battle with a Ditto without an item and a Pokemon holding the item you want to duplicate. Make Ditto transform into the Pokemon holding the item. Switch the Pokemon holding the item with the Pokemon that knows Thief. Use Thief on Ditto. Win the Pokemon battle. The Thief Pokemon now has a duplicate of the item.
Submit a Pokemon in the GTS, and when the little clock shows up at the bottom of the screen, wait until it goes around 7 to 8 times (no way of knowing for sure; you just have to keep trying), and then turn off the system. If you did it at just the right time, it should say the save data has been corrupted (that's fine; there is backup data), and you should have the Pokemon in the GTS and in your PC. Just take back your Pokemon from the GTS, and you now have a clone. It may take quite some time before you time it just right. Also make sure you say when you originally put it in the GTS that you are looking for a Pokemon that no one would have (name a gender, and then say you want a lv100 Abra or wurmple). That way no one can trade before it clones.