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Warrior (original 2006; edition 2008)
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Wikipedia in English (1)
Amazon.com Product Description (ISBN 0446616982, Mass Market Paperback)When a witch is born, a doppelganger is created. For a witch to master her powers, the twin must be killed. But what happens when the doppelganger survives? This debut is an original adventure fantasy about a young witch who must kill her twin in order to survive. Original.
(retrieved from Amazon Wed, 20 Apr 2011 20:03:29 -0400)
"Mirage, a bounty hunter, lives by her wits and lethal fighting skills. She always gets her mark. But her new mission will take her into the shadowy world of witches, where her strength may not be a match against powerful magic. Miryo is a witch who has just failed her initiation test. She now knows that there is someone in the world who looks like her, who is her: Mirage. To control her powers and become a full witch, Miryo has only one choice: to hunt the hunter and destroy her"--P.  of cover.
(summary from another edition)