YA "horror" novels. This woman pretty much had the same plot every book...

It was, as follows:

There's a girl, who has a relatively normal bordering-on-awesome life. But then, some other creepy girl, a lot like her but usually like, HOTTER or something comes along and tries essentially to try and take her life over. (She does everything she does only BETTER, etc.) The circumstances about her origins or how she goes about doing this are generally very bizarre.

The one I remember most specifically was about this Native American girl who finds out she was adopted and that... she has a twin, somewhere, controlling her or something like that? Or trying to, and eventually she succeeds. Also, she has a love interest with half his face burned off. (Happened in a motorcycle-related incident, I think.)

Help, please?

Maybe Christopher Pike's books? I didn't read him very widely, but his Remember Me had a lot of elements similar to what you describe.

No, I don't think that's it (but his work certainly does look interesting, haha!). I /think/ the author was female, but I could definitely be wrong about that. I might have just assumed s/he was female because I felt like the intended audience was definitely for girls.

I remember the cover of the book, when I read it (though, it was old THEN). It was of the girl, and she was looking in the mirror holding something and her reflection was doing something different then she was.

Lois Duncan's Stranger With my Face sounds like the one book you described, she has a string of paranormal/horror books. The first description you gave sounds more like Christopher Pike or R. L. Stine's Fear Street series.

Yes! Lois Duncan, that's who it was! You've done it again, thank you. :)

2/2, sweet success!