DELETE FROM comments c1
WHERE EXISTS (
SELECT *
FROM comments c2
WHERE c2.id <> c1.id
AND c2.name = c1.name
AND c2.email = c1.email
AND c2.comment = c1.comment
)
AND c1.id <> (
SELECT MIN(c2.id)
FROM comments c2
WHERE c2.name = c1.name
AND c2.email = c1.email
AND c2.comment = c1.comment
)