As I recall, there are two rules to shopping for a prom dress:
1) Find a dress that manages to camoflauge any and all signs of teenage female awkwardness while simultaneously accenting any assets the wearer deems worthy of attention; and,
2) DO NOT WEAR THE SAME DRESS AS SOMEONE ELSE.
I'm not sure if the second rule was made to avoid the uniform "bridesmaid look" or if it has more to do with infringement on unique self-expression (my suspicion is the latter). In either case, I had assumed it was a phase that eventually was outgrown.
We held a surprise birthday party for my grandmother and invited several of her closest friends and her one remaining sister. My grandmother and her sister have a true sibling relationship. There is constant jabbing, nit-picking, and unadmitted sincere concern for the other. And, they're polar opposites.
The shock of the surprise (partly due to the fact that she thought we were crashing someone else's party) was nothing compared to the horror she expressed at realizing that her sister was wearing the exact same shirt that she had worn.
Truly the problem was not that her sister was infringing on her unique self-expression. I suspect the problem was more the realization that they might have something in common. Even if it was only an article of clothing.
Perhaps that's the reason for not wanting to wear the same prom dress: one might have something in common with a complete stranger. Oh the horror.