mysql> 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 LIMIT 5;
+----+-------------+----------------+
| id | playername  | ogame_playerid |
+----+-------------+----------------+
|  2 | C3ls1us     |           NULL |
|  3 | gladiatore  |           NULL |
|  9 | Legor       |           NULL |
| 12 | caio gracco |           NULL |
| 18 | Skull       |           NULL |
+----+-------------+----------------+
5 rows in set (0.00 sec)

mysql> SELECT id,playername,ogame_playerid FROM players WHERE playername = 'C3ls1us';
+-------+------------+----------------+
| id    | playername | ogame_playerid |
+-------+------------+----------------+
|     2 | C3ls1us    |           NULL |
| 23795 | C3ls1us    |         100292 |
+-------+------------+----------------+
2 rows in set (0.00 sec)