UPDATE mfng.data d
LEFT JOIN (
SELECT d2.serialpack
FROM mfng.data d2
GROUP BY d2.serialpack
HAVING count(d2.serialpack) >1
) dup
ON d.serialpack = dup.serialpack
SET d.uniquepackserial = CASE
WHEN dup.serialpack IS NULL
THEN 0
ELSE 1
END;