Phixius

Corrected entry: When Mewtwo collects the trainer's Pokemon in the special cloning balls to be cloned, he takes away Ash's Charizard. Why would Mewtwo take away Charizard to be cloned? He already has a clone of Charizard so he wouldn't need another one.

Correction: Being as this is worded as a question, here's the answer: Mewtwo is simply collecting all the Pokémon. He's not taking inventory just then.

Phixius

Correction: Then why doesn't the cloning machine clone another Charizard? If the cloning ball with Charizard went down to the cloning machine then surely that Pokémon would have been cloned.