SELECT `e1`.`user2` FROM `event` AS `e1`
INNER JOIN `event` AS `e2` ON `e1`.`user2` = `e2`.`user1`
WHERE `e1`.`user1` = $user AND `e2`.`user2` = $user
SELECT `e1`.`user1`, `e1`.`user2` FROM `event` AS `e1`
INNER JOIN `event` AS `e2` ON `e1`.`user2` = `e2`.`user1` AND `e1`.`user1` = `e2`.`user2`
WHERE `e1`.`user1` < `e1`.`user2`