Question: can ditto breed with ditto?
Answer: Ditto is a very special Pokémon. ... It can breed with any Pokémon other than the Pokémon in the Undiscovered group and itself [...] In other words: It cannot breed with Pokémon of the Undiscovered Egg Group, to which most legendaries belong, or other Ditto. Note that Ditto belongs to its own egg group.