A patron is looking for a book about a girl who is cloned and her clone does things she wouldn't want her to. The cover features a girl running in two different directions - the top half in one direction and the bottom in the other. This is all I know about the book currently.
This might be part of the Replica series by Marilyn Kaye. The one that sounds the most similar is Another Amy.
Perfect Amy Candler tries hard to blend in with everyone else, but then she comes face to face with someone who looks just like her--someone who is another Amy. The two girls look identical, and Amy expects them to have identical personalities. But she's in for a big surprise. The other Amy is determined to stand out from the crowd. She's determined to grab the spotlight, even if it means squashing the competition. She's determined not to have a look-alike, and she wants Amy out of the way. She's...well...evil! Amy must stop her, if she can.
The cover semi matches your description: