It's been posted here a couple of times, but the site at Howmanyofme.com claims to tell you how many in the US have the same first name, how many have the same last name, and how many have both names the same as the one you enter.

Previous discussions have disputed the accuracy of the reports given by the site, but it's probably close enough for settling small bar bets(?).

There are separate sites for How Many of Me Facebook, and I think I saw one specific for Ireland. There probably are additional ones for other countries (and other affiliations) but I only looked at the Google result far enough to get to the generic one for the US. Searching for "How Many of Me" should get as many deviant sites as you'd be likely to want.

I found that the number of USians with my first name has increased by quite a bit, the number with my last name has decreased slightly, and the number with both names together has decreased rather dramatically, since my last look. (Somebody in the family got pissed with the great great great great great grandfather who had the name combination? - which isn't too surprising, ... or maybe it's because of the couple of mass serial killers with both names.)

John