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Watchers: ID by Peter Lerangis.
The girl's name was Evie, and the cover featured twin girls in green dresses holding flashlights, with lots of disembodied eyes. The black pages also featured the so called "Watchers" discussing the girl's situation, and whether they should or could help her. The Watchers was a book series my brother got into. The books were all pretty much unrelated sci-fi/supernatural stories that had the black pages with the musings of the Watchers, who aren't actually part of the story really, just kind of... watching and commenting. ID was the only one I ever really read, though I skimmed through the Island one.
Pardon my squeal of joy, buy I'm so excited, I've never gotten a YKTS before, and I'm positive this is what you were thinking of!
Edited by Wynne101

This one opens with our heroine as a child finding out that she's adopted. She is not at all happy to hear this and smashes a number of objects (including a couple of potted plants? I want to say violets?).
The next chapter skips ahead several years. Now our heroine is age 13-going-on-14 and is out skiing with a friend. She accidentally collides with another skiier, a fellow teenage girl who turns out to be having a heart attack. Later the girl is reported dead on the news, so our heroine does some internet research and discovers that there's something of an epidemic going on — hundreds of teenagers around the world have been falling ill and dying from medical conditions that usually only affect elderly people — arthritis, hardening of the arteries, heart failure, etc.
While researching, our heroine stumbles across a photograph of one victim who looks exactly like her. Long story short, it turns out that our heroine is a clone, the latest in a series of five clones who were given names beginning with A, B, C, D, and E — with our heroine being "E" — each one born a year apart and each bearing a red arrow-shaped birthmark on the back of her neck. The clones were created by a doctor whose daughter had a gene for the mysterious only-supposed-to-affect-old-people illness, and in an effort to find a cure for her, he created these five clones, each of whom would have the same gene and would give him more chances to try out potential treatments.
The plot focuses on our heroine as she tracks down the other four girls, visiting their homes and talking to their family and friends, only to discover that each one died of the illness. She's also on a time limit, because the illness strikes at age 14, and our heroine only has a few more days before her birthday, so she's getting sicker and sicker as the plot progresses. Eventually she finds the doctor's old house, finds his stash of serums that he made in hopes of curing the girls, and manages to cure herself just in time.
Let's see, a few more details: In between each chapter, there's a black page with white typeface that shows a bunch of numerical data — the current death-toll of the mysterious illness and (I think) the statuses of clones A, B, C, D, and E. I seem to remember that the C clone's name was Caroline. The reading level for this book was probably early teens or possibly pre-teen.
Edited by thesunisup