Well, the only reasons that you would breed another one are that you want Hypnosis or Mirror Coat, for Hypnosis breed a female Milotic (if it's not female, breed it with a Ditto) with a male Politoed, Poliwrath etc. that has Hypnosis. For Mirror Coat, breed a female Milotic with a male Corsola that has Mirror Coat. The other reason is for a better nature, you can Ditto breed for that.
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