The first thing I see is that your friend obviously admires you. You seem to be a person who marches to the beat of your own drummer. This is a trait that most people admire...especially if they don't have the courage to be "different." Are there other things in your life that she looks to you for as well? Your faith? Your funny personality? Do you have an easy time making friends? Does she have other friends besides you?
There's got to be some underlying reason why she's holding onto you so tightly. It sounds like y'all have been friends for quite a while. Perhaps you're even roommates?
Have you tried creating some distance between you? Perhaps encouraging her to develop another hobby...that you're not involved with?
I don't know...this is so tough. I don't believe in brushing people off...which you don't seem to believe in either, or else you would have been out of this friendship a long time ago. But, you are not responsible for her emotional well-being...to the extent that it affects you so much. It's a fine line to walk.
It sounds like she isn't going to change anytime soon. You need to make a decision. What is the friendship worth? Are her annoying habits so distracting from the friendship that lies underneath that you need to end it...at least temporarily.
If you've been friends for a long time, one would think that you could sit down and talk to her. Now, she may not choose to "hear" you, but you need to be honest with her. You could decide to give yourself some space from her. The nice thing would be to sit her down and explain why, rather than giving her the cold shoulder without an explanation. She might not understand at that moment, but she may come to understand it later. That's up to her and whether she's mature enough to grow. Again, that's not up to you.
Another option would be to avoid the situations in which she copies you. Go shopping by yourself or with a different friend. However, I realize that this probably won't totally solve the problem, because she could decide to buy what you came back with.
No easy answers. You're probably finding yourself withdrawing from her gradually...not sharing information with her in fear that she'll jump on the bandwagon too. If that's happening, the communication will wither down to the point where you're not really sharing who you are (because sharing our interests and latest buys are the things that make up our life). Then can you really call it a friendship?
Not easy...I'll pray for you.