This is a really basic query which makes use of the GROUP BY and HAVING clauses:

SELECT lastname, COUNT(*)
FROM persontable
GROUP BY lastname
HAVING COUNT(*) > 1

This query will return all the last names that are duplicated and the number of times their duplicated. If you want to return the entire name, you'll need to use the above query as a subquery:

SELECT firstname, lastname
FROM persontable
WHERE lastname IN (
SELECT lastname, COUNT(*)
FROM persontable
GROUP BY lastname
HAVING COUNT(*) > 1)

-Mike