diff --git a/users/models.py b/users/models.py
index f3e786c0..03d680de 100755
--- a/users/models.py
+++ b/users/models.py
@@ -104,6 +104,13 @@ def linux_user_validator(login):
             params={'label': login},
         )
 
+def pseudo_taken(login):
+    """ Retourne une erreur de validation si le login ne respecte
+    pas les contraintes unix (maj, min, chiffres ou tiret)"""
+    if (EMailAddress.objects
+            .filter(local_part=login.lower())):
+        raise forms.ValidationError('Pseudo is already taken')
+
 
 def get_fresh_user_uid():
     """ Renvoie le plus petit uid non pris. Fonction très paresseuse """
@@ -195,7 +202,7 @@ class User(RevMixin, FieldPermissionModelMixin, AbstractBaseUser,
         max_length=32,
         unique=True,
         help_text="Doit contenir uniquement des lettres, chiffres, ou tirets",
-        validators=[linux_user_validator]
+        validators=[linux_user_validator, pseudo_taken]
     )
     email = models.EmailField(
         blank=True,
@@ -1132,8 +1139,7 @@ def user_post_save(**kwargs):
     Synchronise le ldap"""
     is_created = kwargs['created']
     user = kwargs['instance']
-    EMailAddress.objects.get_or_create(
-        local_part=user.pseudo.lower(), user=user)
+    EMailAddress.objects.get_or_create(local_part=user.pseudo.lower(), user=user)
     if is_created:
         user.notif_inscription()
     user.state_sync()
@@ -1740,6 +1746,9 @@ class EMailAddress(RevMixin, AclMixin, models.Model):
     def __str__(self):
         return str(self.local_part) + OptionalUser.get_cached_value('local_email_domain')
 
+    def clean(self):
+        self.local_part = self.local_part.lower()
+
     @cached_property
     def complete_email_address(self):
         return str(self.local_part) + OptionalUser.get_cached_value('local_email_domain')