I'm particularly interested in famous individuals who have an identical twin who isn't famous. For example, movie star Jon Heder has an identical twin, Dan Heder, who isn't as famous, but I'm not exactly sure why Jon Heder is famous in the first place. (And I'm not sure he is either.) But that situation is relatively uncommon. Either both identical twins are famous or neither one is. And the percentage of famous people who are identical twins appears to be lower than the percentage of identical twins in the population.
I suspect that considerations such as this tend to discourage identical twins from pursuing a lot of careers with steep pyramids of fame.