Duplicate entry '0-0' for key 'PRIMARY'

bonsoir,
quand je veux inserer dé donnée dans ma base il maffiche ce msg "Duplicate entry '0-0' for key 'PRIMARY'"
voici mon code en php:
include('conf.php');
//exécution des requêtes sql_iga:
$sql = "insert into user (ID_USER, NOM, PRENOM, EMAIL, LOGIN, PASS, PROJET, PROFIL, ENTITE)
values ( NULL,'$nom', '$prenom', '$login', '$pass', '$email', '$proj','$prof', '$entite') " ;
$requete = mysql_query($sql) or die( mysql_error() ) ;

et voici ma table:
---------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------+------------------+------+-----+---------+----------------+
| ID_USER | int(11) unsigned | NO | PRI | NULL | auto_increment |
| NOM | char(30) | YES | | NULL | |
| PRENOM | char(30) | YES | | NULL | |
| EMAIL | varchar(40) | YES | | NULL | |
| LOGIN | varchar(40) | NO | | NULL | |
| PASS | varchar(30) | YES | | NULL | |
| PROJET | char(30) | YES | | NULL | |
| PROFIL | char(30) | YES | | NULL | |
| ENTITE | char(20) | YES | | NULL | |
+---------+------------------+------+-----+---------+----------------+