Delete duplicates and keep the newest one in MySQL.
DELETE mytable
FROM mytable
INNER JOIN (
SELECT max(id) AS lastId, someColumnWithDupes
FROM mytable
GROUP BY someColumnWithDupes
HAVING count(*) > 1
) duplic ON duplic.someColumnWithDupes = mytable.someColumnWithDupes
WHERE mytable.id < duplic.lastId;