For small sample sizes that is the best way, but for a more generic and scalable approach I would go with the following

DELETE
FROM tablename
WHERE rowid = (SELECT t.rowid
FROM tablename t,
(SELECT keyval, min(rowid) as keeprowid
FROM tablename
GROUP BY keyval) k
WHERE t.rowid(+) = k.keeprowid
AND k.keeprowid IS NULL
AND t.rowid = tablename.rowid)

Note that I used = rather than IN as the performance difference can be huge. This will work well on almost any table, but will probably work better in this case if a non unique index is applied to the keyval field. The keyval may actually be more than one field, it could if you wish be every field in the table.