I am guessing that a user can post many comments, and a message can have many comments. Consequently, the user column is a foreign key to a users table, and the message_id column is a foreign key to a messages table. You cannot eliminate duplicates of (user, message_id) because a user could post multiple comments to the same message. You could take (comment, date, comment_id) out to a separate table and then have a join table of (user, message_id, comment_id), but that would be unnecessary since you would not be eliminating duplicates anyway.
If so, I believe that your database, at least this part, is in fourth normal form, so there is generally no need to normalise further (and maybe it is in higher normal forms as well, but I am not well studied on those).
