If I buy a dress from Mandee today for a party today...?

Question

how likely is it that someone else will be wearing the same one?

Answer 1

Well, how many people are at that party?  

Answer 2

If Mandees is "THE STORE" that your crowd shops at then most likely someone will have the same dress as you.  Unless, of course the dress just arrived today, then there's a good chance, you will be an original.   You do not offer your age, so I cannot suggest another store that would be appropriate, but I think any small boutiques would offer different more cutting edge clothes.  I  a lot people tend to habitually go the chain stores which can become so boring.