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Warrior and Witch (original 2006; edition 2006)
Witch by Marie Brennan (2006)
References to this work on external resources.
Wikipedia in English
Amazon.com Product Description (ISBN 0446616974, Mass Market Paperback)When a witch is born, a doppelganger is created. Forthe witch to master her powers, the twin must be killed. Or so itwas believed for centuries . . . Once Mirei discovered that mergingwith a doppelganger gave a witch more power than killing them, thewitches at Starfall have been in an uproar (Doppelganger; 4/06). Herheresy goes against centuries of teaching-and the accepted practice ofmurdering their doubles.On a quest to collect the survivingdoppelgangers, Mirei is unsurprised to discover that there are stillwitches at Starfall who side against her and her unorthodox ways. Butworse, her fellow hunter and closest friend, Eclipse, has beencaptured by these same witches-and has no choice but to swear a bloodoath to kill Mirei. If he fails his blood oath, he will die. With thewitches divided, and Starfall itself in the middle of a civil war,Mirei will have to choose between saving her friend and figuring out away to unite Starfall before it's too late.
(retrieved from Amazon Thu, 12 Mar 2015 18:07:37 -0400)
Once Mirei discovered that merging with their doppelgangers gave witches more power than killing them, the witches at Starfall have been in an uproar. Now she has to choose between saving her friend Eclipse and figuring out a way to save Starfall from its own civil war.
(summary from another edition)