Who will win in a match of Ditto vs Ditto?

Question

Ditto can transform into a thing or Pokemon.

Comments

Ditto.

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Dammit you stole my joke

Answer 1

Ditto vs. Ditto.
Ditto would use Transform, the other one would as well.
This would be repeated until both run out of PP for Transform.
Then they'll start to use struggle. Then, based on Crit chance, attack stat, etc. one of them will faint.

Comments

so ditto wins

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doesn't transform give you 5pp of all moves copied from the opposing pokemon, or did they get rid of that?

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Yup... 5 pp

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i tried it myself and my ditto has no pp left

so it struggles
http://bulbapedia.bulbagarden.net/wiki/Struggle