How to pick up only one row per field?

Question

I have the following query:

SELECT userid, message, usergroup 
FROM   mybb_myprofilecomments, mybb_users
WHERE  userid = uid
AND    usergroup NOT IN (5,7)
AND    approved = 1
AND    isprivate = 0
ORDER BY time DESC LIMIT 0,5

Please assume that uid, usergroup are fields in mybb_users table, and userid, message are fields in mybb_myprofilecomments.

It gives following:

But instead of having 2 rows for userid "6551", I want one of 2 rows (older one at 3rd row in terms of time) gets replaced with a row with another userid that is not listed here.

How can I do this? Thank you very much.