mysql> CREATE TEMPORARY TABLE ptemp SELECT p1.id, p1.playername, p1.ogame_playerid FROM players AS p1, players AS p2 WHERE p1.id != p2.id AND p1.playername = p2.playername AND p1.ogame_playerid IS NULL;
Query OK, 1810 rows affected (0.56 sec)
Records: 1810  Duplicates: 0  Warnings: 0

mysql> DELETE FROM players WHERE id IN (SELECT id FROM ptemp);
Query OK, 1614 rows affected (1 min 4.61 sec)