This trick requires a Nintendo WiFi connection. Go into the GTS building with two and five Pokemon in your party, including the one you want to duplicate. Talk to the woman to save the game then let you in. Choose "Deposit Pokemon" and select the Pokemon you want to duplicate. Select a Pokemon that is impossible to have, such as a level 1 Dialga. When it says "Conditions Okay", press A. Watch the little wheel at the bottom right of the screen and turn off your DS when it has spun around eight times. Turn on your DS, load your game, and go back into the GTS. Note: The number of turns required for this to be successful depends on your internet speed and strength of connection. Eight turns is the average so you have to figure out your number of turns yourself by trial and error. To do this first, try eight turns then check the following outcomes. If eight is too early, try eight and a half, then eight and a quarter, etc. After that, every time you want to clone something, use that number of turns. The following are three possible outcomes after turning your DS on again:
A "Game Corrupted" message appeared and when you went into the GTS, it said "Deposit Pokemon". This means that you turned off the DS too early. Try turning your DS off later.
It loaded normally but when you went into the GTS, it said "Summary". This means that you turned off the DS too late. Try turning your DS off later. Also, be sure to take back the Pokemon and exit the GTS before you can try again.
A "Game Corrupted" message appeared and when you went into the GTS, it said "Summary". This means you have successfully duplicated your Pokemon and you should have the Pokemon in your party and in GTS at the same time. Take back the duplicated Pokemon from the GTS and leave. The duplicated Pokemon will also be holding a copy of whatever item the original was holding, so use this trick to also duplicate items such as Masterballs.