Theory No. 1: Maintaining your cool factor means you must be perceived as trend-setting and unique. Imitating another girl's style exactly is a sign that you're losing your edge. That's why Dunst was probably tickled by what was deemed a disaster for Witherspoon — Kirsten wore it first.
Theory No. 2: A girl's dress-up clothes make a statement about her inner self, and especially about her sexuality. What does she value? What is her relationship with her body? So when she shows up at an event wearing the same outfit as someone else, she's immediately linked with this stranger, a person with whom she might not have chosen to connect. Her response, then, can either be hiding and avoiding (like I did when I was 17) — or celebrating the connection and sharing a laugh.
For example, this is how Witherspoon's publicity people responded to the "crisis":
Although it was disappointing that the dress had already been worn, Reese looked beautiful and the bigger deal is Reese won the Golden Globe for her work in 'Walk the Line.' I still love Chanel, my clients still love Chanel, and we are still working with them.If I worked for Witherspoon, I'd have come up with this instead:
Reese was delighted to discover that she'd chosen the same Chanel dress once worn by the lovely and tasteful Kirsten Dunst. "Kirsten and I must have more in common than winning a Golden Globe award,' she announced. "I'm definitely taking that girl to the mall."