Pokémon cloning can be achieved in Generation VI during a link trade with two Nintendo 3DSes.
The success rate may not be too high, resulting in multiple failed attempts, but the glitch is possible.
A timer is recommended for this method.
1. Begin the trade on the opposite 3DS to the one with the Pokémon you want to clone. This 3DS must also have a Pokémon that you don't mind being deleted.
2. Initiate a trade between the Pokémon you want to clone and the Pokémon that will become deleted.
3. After about 4.5 seconds when "Communicating. Please stand by..." on the touch screen and a blue screen on the top screen appears, power off the Nintendo 3DS with the Pokémon you want to clone.
4. You can now power off the other Nintendo 3DS. If successful, both Nintendo 3DSes now have the Pokémon that you wanted to clone.