It has happened. You show up to a party, a wedding, even just going to the movies and there she is, in the exact same dress as you. In the U.P. this is a frequent occurrence. In high school I had a friend that would get so pissed if someone else showed up in the same shirt. You can't get angry over that, there is no avoiding it unless you somehow only shop at boutiques and those are few and far between in the U.P.
At my wedding three girls showed up in the same dress and one totally rocked it. She blew the other two out of the water. You need to be that girl. Mostly she had the perfect body type for this dress. She was a curvy hourglass and it looked perfect in the dress. Of course, not everyone has an hourglass figure. Whatever figure you have, flute-shaped, apple, pear, full-figured, you have to work your advantages and balance everything out. If you are fuller up top don't wear something that will just make 'em look bigger, same with your ass. If you don't have a waist create a waist with a belt or a dress that is more fitted around the waist. Always hold out for the perfect fit. If something doesn't fit you quite right, then move on to something else or be willing or have the ability to fix what doesn't fit well (too big in the bust is something that can easily be fixed. Call on grandma for that one. Length, too long of straps, waist too large, all pretty easy fixes). But if the outfit puts a glaring look on something you don't want highlighted, or the color is just terrible but it fits well, move on.
It's almost all about the fit, but there are other factors. How you wear it. Shoulders back, spine straight, always. It makes you look thinner and confident. A smile always helps, too, unless you can pull off the badass smirk that makes guys wonder what you are thinking about.
Put together an awesome outfit. The fact that you are wearing the same thing as that other girl won't be glaringly obvious if you have belted the dress or added hot heels, or accessorized to perfection. Another thing to keep in mind is that whatever you have picked to wear usually shows something about you, so keep that idea going with whatever you pair with the outfit. Make those other girls look you up from head to toe. Sometimes those looks are uncomfortable, but bare it and know that you look awesome.