The same thing happened with my best friend. Her sister in law and her went shopping for bridal growns for the sil she was got married last year and my friend is getting married this year. Well she just being nice told her sil the dress she picked out and even showed her. Well the sil picked a dress and they went home. Well wedding day came and the dress the sil picked out with my friend wasn't the same dress it was my friends!! And it was even in my friends colors. But my friend just changed her colors and is making sure not to wear the same veil or her hair the same. She said it happens. And didn't get mad. She said it happens and when someone finds the perfect dress you can't change your mind. And I think she's right. The perfect dress for you is gonna be the perfect dress for someone else. But as my friend says, if someone does have your dress just don't tell the groom then it won't be as much of as a surprise. if you don't want him to see it.
Good luck!!