diff --git a/apps/registration/forms.py b/apps/registration/forms.py
index 18d76eb..b533e73 100644
--- a/apps/registration/forms.py
+++ b/apps/registration/forms.py
@@ -22,6 +22,15 @@ class SignupForm(UserCreationForm):
         ],
     )
 
+    def clean_email(self):
+        """
+        Ensure that the email address is unique.
+        """
+        email = self.data["email"]
+        if User.objects.filter(email=email).exists():
+            self.add_error("email", _("This email address is already used."))
+        return email
+
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         self.fields["first_name"].required = True
diff --git a/apps/registration/tests.py b/apps/registration/tests.py
index e43aabc..f732f16 100644
--- a/apps/registration/tests.py
+++ b/apps/registration/tests.py
@@ -119,6 +119,20 @@ class TestRegistration(TestCase):
         self.assertRedirects(response, reverse("registration:email_validation_sent"), 302, 200)
         self.assertTrue(User.objects.filter(email="toto@example.com").exists())
 
+        # Email is already used
+        response = self.client.post(reverse("registration:signup"), data=dict(
+            last_name="Toto",
+            first_name="Toto",
+            email="toto@example.com",
+            password1="azertyuiopazertyuiop",
+            password2="azertyuiopazertyuiop",
+            role="participant",
+            student_class=12,
+            school="God",
+            give_contact_to_animath=False,
+        ))
+        self.assertEqual(response.status_code, 200)
+
         response = self.client.get(reverse("registration:email_validation_sent"))
         self.assertEqual(response.status_code, 200)