Well, I don't think my brain is working on full power right now because I know there's a way to do this with a HAVING clause, but anyway, right now I'd suggest doing two queries. First, grab the passwords:
SELECT passwd
FROM user
GROUP BY passwd
HAVING count(passwd)>1
Then with the results, grab the user id's
Once you've cleaned it up, I'd suggest putting a unique attribute on the field if you want to keep it that way....