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I'm looking for a book series in which a girl meets aliens. She was the only person on Earth to have met aliens.

One of them was an intelligent purple ball that I think could morph into other shapes, but I could be wrong. She helped them (possibly by fighting aliens or something) and when she was sent to some sort of alien training academy they made her a robot to temporarily take her place on Earth.

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It's the Akiko series, with the storyline you're discussing coming specifically from Akiko and the Journey to Toog.

It's a comic series, but Wikipedia says this was one of the comics adapted to a children's book. It has a purple ball alien, Poog, with the eponymous planet of Toog being were Poog comes from, and one of the plot points is that Akiko needs to get home before her robotic duplicate is discovered.

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