Quite often, identical twins tend to be very similar and have similar interests (not always, but often). One of the hardest parts about becoming successful in anything is having someone to support you and give you the push you need at times. Twins often provide that support for each other. Instead of a "me trying to conquer the world" approach that most of us feel, it is an "us trying to conquer the world".
I ran with a set of identical twins in high school that were good at everything (class president, valedictorian, state qualifiers in XC and downhill skiing, both are now doctors). They are both very high goal setters and thrived off a friendly competition with each other. I'm pretty certain that had they been a "single", they wouldn't have been as successful.
On the other hand, I had a set of identical twins that I coached a few years back. They tended to push each other and became a lot better (both ended up being state qualifiers in XC) as they got older. However, by their senior year, one of them got very complacent and was just kind of floating through his senior year. The other brother was still working hard to improve and started to break away from his brother in workouts (they had always run together). After a few weeks, the faster one slowed down in workouts and started running with his brother again. I found out later that the slower one had been harassing his brother to stop making him look bad.
The twin thing can work both ways. They can help each other rise to heights they wouldn't be able to do alone or they can hold each other back. I also had a an identical twin that ran for me whose sister didn't run at all. Although they looked alike and had similar values, their personalities and interests were very different. When it comes down to it all, although twins are genetically the same, they are still individual people.