diff --git a/authens/backends.py b/authens/backends.py
index 5481a0d..6033aad 100644
--- a/authens/backends.py
+++ b/authens/backends.py
@@ -85,7 +85,7 @@ class ENSCASBackend:
         i = len(cas_login) - 1
         while i >= 0 and cas_login[i] in "0123456789":
             i -= 1
-        radical = cas_login[:i + 1]
+        radical = cas_login[: i + 1]
 
         # Find an integer i such that radical + str(i) is not taken.
         taken = UserModel.objects.values_list("username", flat=True)
diff --git a/authens/tests/cas_utils.py b/authens/tests/cas_utils.py
new file mode 100644
index 0000000..089ffb7
--- /dev/null
+++ b/authens/tests/cas_utils.py
@@ -0,0 +1,19 @@
+class FakeCASClient:
+    """Fake CAS Client that mimics CAS interactions.
+
+    By default, always log in the same user 'johndoe' without checking the ticket.
+    """
+
+    def __init__(self, cas_login="johndoe", entrance_year=2019):
+        self.cas_login = cas_login
+        self.entrance_year = entrance_year
+
+    def verify_ticket(self, ticket):
+        """Dummy client that always log in the same user."""
+
+        attributes = {
+            "homeDirectory": "/users/{}/info/{}".format(
+                self.entrance_year % 100, self.cas_login
+            )
+        }
+        return self.cas_login, attributes, None
diff --git a/authens/tests/test_backend.py b/authens/tests/test_backend.py
index a756f1f..2cf1f39 100644
--- a/authens/tests/test_backend.py
+++ b/authens/tests/test_backend.py
@@ -1,17 +1,19 @@
-from django.contrib.auth import get_user_model
+from unittest import mock
+
+from django.contrib.auth import authenticate, get_user_model
 from django.test import TestCase
 
 from authens.backends import ENSCASBackend
+from authens.models import CASAccount
+from authens.tests.cas_utils import FakeCASClient
 
 UserModel = get_user_model()
 
 
-class TestBackend(TestCase):
-    def setUp(self):
+class TestCASBackend(TestCase):
+    def test_usernames_uniqueness(self):
         UserModel.objects.create(username="toto")
         UserModel.objects.create(username="toto2")
-
-    def test_usernames_uniqueness(self):
         backend = ENSCASBackend()
         for _ in range(10):
             username = backend.get_free_username("toto")
@@ -22,4 +24,69 @@ class TestBackend(TestCase):
             self.assertFalse(UserModel.objects.filter(username=username).exists())
             UserModel.objects.create(username=username)
 
-    # TODO: https://git.eleves.ens.fr/klub-dev-ens/authens/issues/4
+    @mock.patch("authens.backends.get_cas_client")
+    def test_cas_user_creation(self, mock_cas_client):
+        # Make `get_cas_client` return a dummy CAS client for testing purpose.
+        fake_cas_client = FakeCASClient()
+        mock_cas_client.return_value = fake_cas_client
+
+        # First authentication
+        user1 = authenticate(None, ticket="dummy ticket")
+        # Should create a user and a Clipper instance.
+        self.assertEqual(CASAccount.objects.count(), 1)
+        self.assertEqual(UserModel.objects.count(), 1)
+        clipper1 = CASAccount.objects.get()
+        self.assertEqual(clipper1.user, user1)
+        self.assertEqual(clipper1.cas_login, fake_cas_client.cas_login)
+        self.assertEqual(clipper1.entrance_year, fake_cas_client.entrance_year)
+
+        # Second time we authenticate the same user.
+        user2 = authenticate(None, ticket="dummy ticket")
+        # Should return the same user and should not create anything.
+        self.assertEqual(CASAccount.objects.count(), 1)
+        self.assertEqual(UserModel.objects.count(), 1)
+        clipper2 = CASAccount.objects.get()
+        self.assertEqual(clipper1, clipper2)
+        self.assertEqual(user1, user2)
+        self.assertEqual(user2, clipper2.user)
+
+    @mock.patch("authens.backends.get_cas_client")
+    def test_name_conflict_handling(self, mock_cas_client):
+        # Make `get_cas_client` return a dummy CAS client for testing purpose.
+        fake_cas_client = FakeCASClient()
+        mock_cas_client.return_value = fake_cas_client
+
+        # Create a regular user (without a CAS account) with the same username as the
+        # CAS user before her first login.
+        regular_user = UserModel.objects.create_user(username=fake_cas_client.cas_login)
+
+        # First login of the CAS user.
+        cas_user = authenticate(None, ticket="dummy ticket")
+
+        self.assertNotEqual(regular_user, cas_user)
+
+    @mock.patch("authens.backends.get_cas_client")
+    def test_cas_conflict_handling(self, mock_cas_client):
+        # Make `get_cas_client` return a dummy CAS client for testing purpose.
+        fake_cas_client = FakeCASClient()
+        mock_cas_client.return_value = fake_cas_client
+
+        # Create an old CAS user with the same cas_id as the new CAS user before her
+        # first login. This tests the behaviour of the library in the (unlikely)
+        # scenario where the SPI decides to give a clipper login that used to be someone
+        # else's to a new user.
+        old_user = UserModel.objects.create_user(username=fake_cas_client.cas_login)
+        old_clipper = CASAccount.objects.create(
+            user=old_user,
+            cas_login=fake_cas_client.cas_login,
+            entrance_year=fake_cas_client.entrance_year - 10,
+        )
+
+        # Log the new 'johndoe' in.
+        new_user = authenticate(None, ticket="dummy ticket")
+        new_clipper = new_user.cas_account
+        # Check that it gets a fresh user and a fresh clipper account.
+        self.assertNotEqual(old_user, new_user)
+        self.assertNotEqual(old_clipper, new_clipper)
+        self.assertEqual(new_clipper.cas_login, fake_cas_client.cas_login)
+        self.assertEqual(new_clipper.entrance_year, fake_cas_client.entrance_year)