This caculates 2 People having the same birthday in a group of people. It's also generic and should also calculate the chances for a repeat of any two items in a certain size sample set of items based on how many items might be used. I made to so that I could see what the chance for repeat is if you use the last 4 numbers of a social security number as a unique identifier for a certain number of people.

Here is a site that gives details about this.

Sample Size
People

Odds for a repeat with Sample Size 365 and People 30 is 70.63 %