How many people do you need to know before it's likely that two have the same full name?
The answer is going to depend critically on the pool of people you are sampling from, and whether you count people in multiple generations as Sr., Jr. as "same name". Similarly, middle initials or middle names will be distinguishing marks. What I'm after is less about collisions within a family and more about two John Jones in your addressbook, otherwise completely unrelated.
I suspect that the number is in the low thousands, just from personal experience.
How would you approach this problem?
Edward Vielmetti's Wordpress weblog 
Interesting notion. I don’t know how to solve it because there are so many “it depends” – including your geography, your circle of connections and more. And what do you mean by people you know? My address book is filled with people who I’ve heard from – media relations types and career coaches and more yet I don’t necessarily know them.
Maybe one way to start considering this problem is by picking a common name in your circle – since clearly Jennifer Smith will have a different answer than say Vickie Elmer.