They're definitely rare, far more rare than many of the similar things to them like their opposite (chimerism where multiple sets of DNA but one person) or fraternal twins. But there's a lot of people and identical twins have advantages for child acting especially but acting in general. Then consider that likelihood of identical twins is a genetic trait and getting a break in acting is much easier if you have family in the industry.

Long story short, it's rare enough that even if you know several twins you may not know any identicals, but common enough that it's not weird for random actors to have an identical twin. It's typically just a lucky thing though and they rarely cast around it except for children (labor laws)