From Bulbapedia, the community-driven Pokémon encyclopedia.
Cloning glitches are glitches in the Pokémon games that enables a player to make an exact copy or "clone" of a Pokémon. While a cloning glitch has been present in at least one game of each generation so far (except Generation V), 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 only a portion of data have been saved successfully. 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 Generations 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 vice versa 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 vice versa) 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.
Box change 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 (ideally, before the "S" in the dialog Box appears but after the Box has appeared) 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.
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Storage system move method
The player can also exploit the move function within a box to clone a Pokémon. The box must not be full or it will cause the Pokémon to disappear. The player should select the Pokémon he or she wishes to clone. Select move. Move it to somewhere else within the box. During the time that it is saving, the player should count three to five seconds then disconnect the power. This will result in a clone.
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 six Pokémon can be cloned at one time. However, unless you have a Pokémon that you are willing to lose, you can only clone five. It is also possible to clone numerous items at once.
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.
When a Pokémon is successfully transferred to the Pokéwalker but the game is turned off before saving, the Pokémon would appear in both the Pokéwalker and the PC Box it was in prior to the transfer. However, attempting to bring the Pokémon in the Pokéwalker back into the game would cause it to be released into the wild, even if the original Pokémon does not exist in the game any more, such as being traded or released. This is because the data of the Pokémon that was transferred was not saved onto the game, thus the game will not recognize the Pokémon that is on the Pokéwalker and release it. (The game only recognizes Pokémon caught from the Pokéwalker and the Pokémon's data that is stored in the game after the transfer. This is to prevent external or third-party devices from 'sending' Pokémon onto the game.)
In Generation IV, if a player is trading Pokémon and one of the Nintendo DS systems is turned off at a certain time, the Pokémon that was traded to the game that was not turned off will appear in both player's parties. However, this glitch is very difficult to perform and quite commonly corrupts save data. There are reports that this method is effective in the Generation V games, but such reports are unconfirmed.
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.
In Generations IV and V, by manually specifying the DNS address so as to connect to a server other than the GTS, it is possible to clone Pokémon.
*Indicates glitches which have been officially acknowledged