In case OP is still reading - I have been through 3 proms - DD1 is an individual for both her Y11 and Y13 proms and had short fitted dresses, did her own hair, make up and nails, looked great, spent very little and had a fun time. Her choice.
For balance DD2 is more of a sheep, and likes to conform, for her Y11 prom the school had a facebook page and each girl posted their dress on it as soon as they had chosen, the idea being that this would lead to only 1 of each dress. However they still had duplicates and similar styles/colours. They still all had a good time, the pictures were great and they all managed to look individual with their hair and accessories.
For reference even though DD2 has expensive tastes her max budget was £250 and she eventually bought a lovely Coast dress for under £200. Her rational was that it looked great, fitted well and she should be able to wear it again.
There is always a lot of prom hype now, then we found it quietened down as they started to get into revision mode.
Would DD have been annoyed if her friend copied her dress, probably not but she had the confidence that her dress looked great on her and had a good relationship with her friendship group, where 2 of them did chose the same dress but in different colours. Would she have been annoyed if her frenemy chose the same dress, yes of course, would she have change, no because she is stubborn and knows that my patience for shopping had run out.