simple insert in empty table doesn't fail but doesn't insert

CLIENT:
python 3.1.1 + mysql connector 0.1.3
Linux pixel 2.6.31.3-nogo-pixel #1 SMP PREEMPT Mon Dec 7 01:14:18 EST 2009 i686 Intel(R) Core(TM)2 CPU U7600 @ 1.20GHz GenuineIntel GNU/Linux

SERVER:
Server version: 5.1.39-ndb-7.0.9-cluster-gpl-log MySQL Cluster Server (GPL)
Linux oneohone 2.6.18-6-amd64 #1 SMP Fri Dec 12 05:49:32 UTC 2008 x86_64 GNU/Linux

COMMAND:
MySQLCursor.execute( query = "INSERT INTO users (`username`) VALUES (%s) ON DUPLICATE KEY UPDATE `username`=VALUES(`username`)", args = ['SYSTEM'] )

Debug from within cursor.py:
cursor.MySQLCursor.execute(...): 1 BEGINNING
cursor.MySQLCursor.execute(...): 2 AFTER _reset_result() operation == 'INSERT INTO users (`username`) VALUES (%s) ON DUPLICATE KEY UPDATE `username`=VALUES(`username`)'
cursor.MySQLCursor.execute(...): 3a == params is not None
cursor.MySQLCursor.execute(...): 4 stmt == 'INSERT INTO users (`username`) VALUES ('SYSTEM') ON DUPLICATE KEY UPDATE `username`=VALUES(`username`)'
cursor.MySQLCursor.execute(...): 5a _have_result == 'False'
cursor.MySQLCursor.execute(...): 6 _executed == 'b"INSERT INTO users (`username`) VALUES ('SYSTEM') ON DUPLICATE KEY UPDATE `username`=VALUES(`username`)"' rowcount == '1'

As I mentioned in the summary: It is an empty table (below you will find the show create table).
After several trials I then copied and pasted the "_executed" string in the mysql client on the server and here is what I saw:
  + at the beginning the table was still empty!
  + then I tried the _execute string (in the mysql client) and selected and got an ID of 12 !!?? (.. and I did not use the table in any other way --> I guess the "insert trials" from python did increase the ID (without inserting anything))
 + then I ran the python program again 3 times
 + then I "DELETE"d (in the mysql client)
 + and ran the "INSERT" again (in the mysql client) and got an ID of 16

Why is this not working? Am I missing something?!
Any help very appreciated.
Please let me know if you need any further information.

mysql> use user;
Database changed
mysql> select * from users;
Empty set (0.00 sec)

mysql> INSERT INTO users (`username`) VALUES ('SYSTEM') ON DUPLICATE KEY UPDATE `username`=VALUES(`username`);
Query OK, 1 row affected, 1 warning (0.00 sec)

mysql> select * from users;
+----+----------+----------+----------+
| ID | username | password | creation |
+----+----------+----------+----------+
| 12 | SYSTEM | | NULL |
+----+----------+----------+----------+
1 row in set (0.00 sec)

mysql> delete from users where username = 'SYSTEM';
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO users (`username`) VALUES ('SYSTEM') ON DUPLICATE KEY UPDATE `username`=VALUES(`username`);
Query OK, 1 row affected, 1 warning (0.00 sec)

mysql> select * from users;
+----+----------+----------+----------+
| ID | username | password | creation |
+----+----------+----------+----------+
| 16 | SYSTEM | | NULL |
+----+----------+----------+----------+
1 row in set (0.00 sec)

mysql> show create table users;
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| users | CREATE TABLE `users` (
  `ID` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `username` char(128) COLLATE utf8_unicode_ci NOT NULL,
  `password` char(128) COLLATE utf8_unicode_ci NOT NULL,
  `creation` timestamp NULL DEFAULT NULL,
  PRIMARY KEY (`ID`),
  UNIQUE KEY `ID_UNIQUE` (`ID`)
) ENGINE=ndbcluster AUTO_INCREMENT=17 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)