Prom horrors

I bought a prom dress today and I fell in love. Unknown to me another girl going has the same dress. I plan on changing it around a lot and can't take it back. I could care less and even think it's kinda cool (we look nothing alike body wise at all) but she's making a big deal out of it. There will be 300+ people there and we don't hang out at all so I just can't see it being an issue. I said I was sorry but she won't shutup about it. I paid 700$ for this dress and I finally feel good about my body. I just don't know what to do.