My brother-in-law's girlfriend wore an almost identical dress as me at a wedding and now I'm very mad.
Why does wearing the same dress irritate you?
It was a fancy fancy dress party that I wore an expensive dress for. She looks exactly like me thus my dress that I spent so much on went to waste.
It's a wedding, so you aren't the focus and neither is the girlfriend. You need to be happy at the wedding, the spotlight isn't on you. You can't dictate what other people can and can't wear, it's impossible.
But what if I did spend so much on my dress to look different.
You may find it to be annoying, but the wedding is about the bride and not you. You can't be too upset if you weren't special on this day. During your wedding, you will be the most special and that's when no one else should wear the same thing as you.