One day at a time

select address, count(address) as cnt
from mailing_list
group by address
having cnt > 1
order by cnt;

If you are using rails 3.0.0 beta and authlogic, you may see this after you add authlogic in your GemFile. This is a bug in authlogic, you need to disable the authlogic from the GemFile and install authlogic latest plugin as well. The latest authlogic build fixed the problem.