What are the birth places of all the people?
select birth_place from people
How many people in each of the birth place?
select birth_place, count(*) from people group by birth_place
Keep the birth places where the birth place is only token by one person.
select birth_place from people group by birth_place having count(*) = 1
What about at least two people?
select birth_place from people group by birth_place having count(*) >= 2