I have 1 friend who is this way and it drives me batty. Not just because she copies, but because it feels like she is competing and she will try and go buy the item ASAP in order to not fall behind me.
She says she feels intimidated when her friends have better, newer things than she does so she has to keep up. It's one thing to keep up, its entirely different thing to continue to purchase the EXACT same items.
The first few times it didn't bug me, but even my bf has become annoyed and feels she is like the character from Single White Female. Soon as I get something new, she says omg, I have to have it. Next thing you know, she has it. Which is really bad because we hang out in the same social circle so we either end up having the same items on (purses, jewelry, shoes) or I have to double check with her so we don't continue to show up with the same accessories. SO annoying.