Okay, now that I know she is not 15 I can see where this is getting really
old! However, it seems to me that she's very insecure in her own choices and for various reasons she has attached herself to you. She may feel you're prettier/smarter/more popular or whatever and in some way feels that by being like you will bring her the same thing. We know this doesn't make sense, but psych issues often don't.
You could try what Nonny2 suggested to make her see what is going on, but based on what you said I think she knows. She just can't help herself. You might try the shopping thing and help her to find her own
best look, the colors that look best on her. Maybe you could both take one of those classes that help people find the colors/style they look best in. Of course if she has the same coloring/body type as you this may not work. I don't think there are any easy answers. You have to decided how much effort you want to put into this relationship.