By Marie Brennan.
Mirage is a Hunter by profession. She and her friend Eclipse get a job investigating the murder of a witch. This isn't normally the sort of thing Mirage would do-- as a natural redhead, people have been thinking she's a witch for years and boy, is she tired of it-- but oddly enough, she's willing to take this job. However, it turns out that this job is relevant to her personally...
Miryo is a witch, who's finally coming into her powers at age 25. She passes her witch test...but then something goes wrong. That's when she finds out that when a witch is born, a non-magical doppelganger of her is created-- and that doppelganger has to die in order for a witch's powers to work correctly without exploding and killing her/anybody in the vicinity. However Miryo's doppelganger managed to survive, the top witches tell her that she needs to murder her doppelganger in order to have her magic work. Miryo never wanted to be a killer in the first place and isn't comfortable with this, but goes to seek out her double anyway.
Happily for me, that scenario isn't drawn out too much. When they meet, Miryo can't bring herself to do it. And Mirage is all, "Yeah, I couldn't kill me either." So the two of them form an alliance to try to figure out some other way to solve their problem without resorting to murder. They try various experiments, but so far nothing's worked. And apparently the top witches are REALLY against them trying anything other than the solution that's been "working" for ages. (I especially liked the twist as to how this situation came about.)
I liked this story-- despite being doubles, the two ladies come across as different yet similar personalities well enough, and I liked the plotting of it. It almost seems like the solution worked out coincidentally easy to me, but given the worldbuilding that doesn't seem out of character. I enjoyed it. Four stars.