Rarity and the Curious Case

Can Rarity and her pony friends solve the curious case of Charity? When Rarity welcomes her new apprentice, Charity, to Ponyville, she is sure that they will become best friends. But when Charity dyes her mane and tail to look more like Rarity, and begins to copy her every move, Rarity realises this pony is a problem! Can Rarity solve the mystery of Charity's true identity?