I'm upset that both of my roommates bought the exact same shoes as me
This is not necessarily a bad thing! It could simply mean that you both have good taste.
The shoes are quite expensive, I felt sad that they didn't ask me first to choose ahead of time.
Rather than being upset, you should feel flattered that they liked your shoes so much they wanted them too! Roommate issues can happen quickly and you shouldn't let something small like this get in the way of a good situation.
I rarely share things with my roommate so it's only logical that I will feel upset.
Well, I do see where you are coming from, but you aren't sharing the same shoes. You both have your own, it's just the style is the same. Like I said before, they are copying you because they see you as the expert and have good taste in fashion.