From Bulbapedia, the community-driven Pokémon encyclopedia.
Pokémon cloning is a glitch in the Pokémon games that enables a player to make an exact copy or "clone" of a Pokémon. While cloning has been present in at least one game of each generation so far, the methods to achieve it vary from game to game. Most of these methods involve exploiting the relatively long save times of the Pokémon games to interrupt the saving process when some data has been changed and some still hasn't. Like most glitches, there are negative side effects, such as the potential loss of the Pokémon to be cloned or even corruption of the saved data.
Generations I and II
There are two different methods for cloning Pokémon in the games from Generation I and II, one involving a Pokémon trade and the other the use of the storage system. Even though both are possible in all of the handheld games of these two generations, the trading method is mostly associated with Generation I, and the storage system method with Generation II.
The cloning glitch most popular in Generation I involves the interruption of the trading process. As link cables for the Game Boy are unable to send and receive data simultaneously, one game cartridge first sends its traded Pokémon's data while the other receives it, and then viceversa for the second traded Pokémon. Interrupting the trade after one Pokémon was sent and the other wasn't causes the first Pokémon to be present on both games and the second one not present on either, essentially making two of the same Pokémon while making the other disappear. The interruption can be either accidental - because of dirty connectors or damaged link cables - or on purpose - by disconnecting the link cable or turning the Game Boy off.
In the Generation I games, this glitch is potentially very hazardous to the save file, because in the period of time between sending and receiving a Pokémon (or viceversa) the game saves the player's party. If the glitch is performed wrong and the game has not finished saving, the saved data will be corrupted. In Generation II this process was changed, and thus this method is not hazardous for the save file.
Storage system method
Another cloning method present in the early Pokémon games involved the exploit of the Pokémon storage system and the fact that the game needed to save every time a box was changed. During the saving that takes place when switching the box, first the Pokémon data on all the PC boxes is saved, and then the Pokémon data on the player's party. If some Pokémon are deposited in the PC before switching the box, interrupting the saving process (turning the power off) at the right moment causes said Pokémon to be present both in the box and the player's party. This method, unlike the trade method, allows the player clone up to five Pokémon at the same time, as well as in Generation II, clone items by making the cloned Pokémon hold items before cloning it.
If the glitch is used with a game attached to Pokémon Stadium 2, there is a chance of getting strange glitch Pokémon. If the player withdraws a Pokémon and continues the cloning glitch, if the game is turned off at the right time, the cloned Pokémon will vanish and be replaced by a Pokémon with the same name, attack, and level as the Pokémon removed. Its sprite will be identical to the cloned Pokémon, but if sent into battle, it will appear as the withdrawn Pokémon. Its species name will be the same as the cloned Pokémon's and it will be able to learn all the TMs/HMs as the cloned Pokémon. However, it will evolve as the withdrawn Pokémon does.
This glitch may be used at the beginning of a game to obtain all three starter Pokémon.
Due to improvements of the games and hardware such as the capability of the Game Boy Advance link cable to send and receive data at the same time, the auto-canceling of trades if something goes wrong, the lack of need to save while changing PC boxes and new data corruption protection, both methods exploited in the first two generations to clone Pokémon were essentially removed for the third. While Pokémon cloning was completely absent on most games of this generation, the glitch returned in a different form in Pokémon Emerald, where the Link-Battle mode of the Battle Tower saved only the party and bag of the player, even if he or she chooses "no" when the game asks to; this was due to the game's need to save a massive amount of data and check connections. Like in the methods from the previous generation, if the save is interrupted at the right moment, up to five Pokémon can be cloned at one time. It is also possible to clone numerous items at once.
Another glitch occurs when trading between any GBA game and Pokémon Colosseum and Pokémon XD that can be used to clone Pokémon. Unlike the handheld games of that generation, Colosseum and XD does not require the user to save the game after a trade is conducted. To enable the glitch, the player has to have Pokémon in the Game Boy game that he/she is willing to lose. After the trades have been completed, the GameCube will have to be shut down or reset without saving. The Pokémon traded to the Game Boy game will appear in the party of the GameCube game, while the same Pokémon will appear in the Game Boy party.
In Generation IV, a cloning method reminiscent of the methods from the first two generations appeared, involving the use of the GTS. If the Pokémon is deposited in the GTS and the connection is interrupted at the right time, the Pokémon in question would be in the player's party or PC box and in the GTS, requiring the player only to withdraw the Pokémon from the GTS to obtain the clone. In Pokémon Platinum Version, however, the glitch of cloning via GTS was removed; if it is attempted, the game simply reports a communication error and returns the player back to the main desk.
If the power is shut off on the DS after a Pokémon is transferred to the Pokéwalker before the game begins to save, the transfer is not recorded on the game and the Pokémon will still be in the PC Box it was in prior to transfer. This glitch may also delete the Pokémon if it was being transferred from the Pokéwalker to the game. However, the game will automatically delete the clone if the player attempts to bring it back onto the game (even if the original Pokémon does not exist in the game any more, such as being traded).
Corrupted save file
Sometimes, when a game's save file gets corrupted, the Pokémon in that save file are kept. When this happens, the game can still be saved, but the save file will be unable to be loaded. With the assistance of another Nintendo DS and Generation IV game, the Pokémon can be traded from the corrupted game. After the trade is completed, if the game is reset, the Pokémon traded from the corrupted game will be in both games, but the other traded Pokémon will be lost.