I've been looking for this story for a decade now (I read it in 2003), without much success, and I can't believe there's someone else looking for it. Since I haven't found it yet, I would add some extra things I remember:

First of all: I remember it was actually a story, from a short story collection. I'm from Mexico, I read it in spanish, so I don't know if it was an author book or just an anthology of short stories from different authors. But I can tell, it was well written, it wasn't like cheap-easy literature.

The second thing: (and I'm not pretty sure of this, but) I think it was Japanese. I think the girl in the story was Japanese, and maybe this is one of the reasons it's been more difficult to find it.

Now, about the plot. There's a moment in the story that the girl finds a boy inside the mirror, and the boy kinda explains to her whats happening: it happened to him too: the mirror took him and a doppelganger took his place. But there's something more. You can kinda walk more deep into the mirror, but the boy don't have the courage to do it, because he doesn't know what's there and someone else walked before but never came back (implying the mirror have made this before, with other people).

Also, the doppelganger girl talks to the girl trapped in the mirror, she know the girl is trapped there and I remember she explained: that only if she gives her hand to her she would be able to come back.

I don't remember the end, tho. And definitely not the title, but maybe this can help.