Onlamp provides a very helpful page about identifying 'non'-distinct
values within a table (see

They summary the process with an example as follows:

mysql> CREATE TABLE tmp
-> SELECT COUNT(*) AS count, last_name, first_name
-> FROM cat_mailing GROUP BY last_name, first_name HAVING count >
mysql> SELECT cat_mailing.*
-> FROM tmp, cat_mailing
-> WHERE tmp.last_name = cat_mailing.last_name
-> AND tmp.first_name = cat_mailing.first_name
-> ORDER BY last_name, first_name;

Just out of curiousity, can this be reduced down to a single query?