Question

First who would win? And what would happen?

I always like imagination.

Answer 1

Since Generation 2, you cannot use Transform twice in a battle. So if Ditto transformed into Mew or vice-versa, neither pokemon can use Transform again.

In Generation 1, if Mew transformed into Ditto first, then Transform now will just turn Ditto into Ditto. It would be a never ending battle because they only know Transform and the PP never runs out.

If Ditto transformed into Mew first then you'd have 2 Mews. If either uses Transform they stay as Mew (but their PP is reset).

Comments

Would it just fail?

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Pretty much.

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I would like to see someone do a battle like that. :)