Multiple correct submissions cause SQL exception about mellivora HOT 4 CLOSED

$challenges = db_query_fetch_all('
    SELECT
       c.id,
       c.title,
       c.description,
       c.available_from,
       c.available_until,
       c.points,
       c.num_attempts_allowed,
       c.min_seconds_between_submissions,
       c.automark,
       c.relies_on,
       IF(c.automark = 1, 0, (SELECT ss.id FROM submissions AS ss WHERE ss.challenge = c.id AND ss.user_id = :user_id_1 AND ss.marked = 0)) AS unmarked, -- a submission is waiting to be marked
       (SELECT ss.added FROM submissions AS ss WHERE ss.challenge = c.id AND ss.user_id = :user_id_2 AND ss.correct = 1) AS correct_submission_added, -- a correct submission has been made
       (SELECT COUNT(*) FROM submissions AS ss WHERE ss.challenge = c.id AND ss.user_id = :user_id_3) AS num_submissions, -- number of submissions made
       (SELECT max(ss.added) FROM submissions AS ss WHERE ss.challenge = c.id AND ss.user_id = :user_id_4) AS latest_submission_added
    FROM challenges AS c
    WHERE c.category = :category
    ORDER BY c.points ASC, c.id ASC',
    array(
        'user_id_1'=>$_SESSION['id'],
        'user_id_2'=>$_SESSION['id'],
        'user_id_3'=>$_SESSION['id'],
        'user_id_4'=>$_SESSION['id'],
        'category'=>$current_category['id']
    )