What about the Doppleganger? (SPOILER)

What I don't get is why her doppleganger tried to kill her, and failed.

Did all the others have dopplegangers, too? Seems like they had internal parasites, as they first gave her, then it was just a dream, then they sent her look-alike to kill her. Then that one dies, but SHE gets sent to the ship, then back (still wounded), starts to laugh, and...

Maybe these are just plot holes. As an author, I know sometimes those slip through. Or some writers just don't care about such "trivia."