id | from | to
------------------
1 | John | Robert
2 | John | Michael
3 | Robert | John
4 | Michael | John

John | Robert
John | Michael

SELECT `from`,`to` FROM s_msgs WHERE id IN(

             SELECT id FROM (
                 SELECT  MIN(id) AS id,
                    CASE
                        WHEN STRCMP(`to`,`from`) = -1 THEN CONCAT(`to`,`from`)
                        ELSE CONCAT(`from`,`to`)
                    END
                    AS conc
                FROM s_msgs

                GROUP BY conc

                ) AS t
            )

SELECT DISTINCT `from`, `to`
FROM YOUR_TABLE
UNION
SELECT DISTINCT `to`,`from`
FROm YOUR_TABLE

SELECT *
FROM   MY_TABLE f
WHERE  NOT EXISTS (
                    SELECT 1
                    FROM MY_TABLE t
                    WHERE f.`from` = t.`to`
                    AND f.`to` = t.`from`
                    AND f.id > t.id)