This problem is a favorite in the probability literature. In a group of 100 people, what is the probability that at least two have the same birthday?
Yes, that is a sequence of six 9s followed by a 7! Hence, we can be almost certain that any group of 100 people has at least two people sharing birthdays.
A similar calculation reveals the probability of a shared birthday already exceeds 1/2 at 23 people, a quite surprising result.
About 50% of school classes thus ought to have kids who share birthdays, something that those with idle time on their hands can check empirically.