diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9e8b98ee..0d801a02 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,10 @@ and this project adheres to
 
 - 🔧(backend) backport logging configuration from docs
 
+### Fixed
+
+- 🐛(backend) prevent duplicate pending users on concurrent requests
+
 ## [1.17.0] - 2026-05-31
 
 ### Added
diff --git a/src/backend/core/migrations/0019_user_unique_email_when_sub_is_null.py b/src/backend/core/migrations/0019_user_unique_email_when_sub_is_null.py
new file mode 100644
index 00000000..154128fe
--- /dev/null
+++ b/src/backend/core/migrations/0019_user_unique_email_when_sub_is_null.py
@@ -0,0 +1,19 @@
+# Generated by Django 5.2.14 on 2026-06-02 17:31
+
+import django.db.models.functions.text
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('auth', '0012_alter_user_first_name_max_length'),
+        ('core', '0018_rename_active_application_is_active'),
+    ]
+
+    operations = [
+        migrations.AddConstraint(
+            model_name='user',
+            constraint=models.UniqueConstraint(django.db.models.functions.text.Lower('email'), condition=models.Q(('sub__isnull', True)), name='unique_email_when_sub_is_null'),
+        ),
+    ]
diff --git a/src/backend/core/models.py b/src/backend/core/models.py
index 45fb271f..32d17db8 100644
--- a/src/backend/core/models.py
+++ b/src/backend/core/models.py
@@ -211,6 +211,13 @@ class User(AbstractBaseUser, BaseModel, auth_models.PermissionsMixin):
         ordering = ("-created_at",)
         verbose_name = _("user")
         verbose_name_plural = _("users")
+        constraints = [
+            models.UniqueConstraint(
+                models.functions.Lower("email"),
+                condition=models.Q(sub__isnull=True),
+                name="unique_email_when_sub_is_null",
+            )
+        ]
 
     def __str__(self):
         return self.email or self.admin_email or str(self.id)
diff --git a/src/backend/core/tests/authentication/test_backends.py b/src/backend/core/tests/authentication/test_backends.py
index 4e6d649d..2f707120 100644
--- a/src/backend/core/tests/authentication/test_backends.py
+++ b/src/backend/core/tests/authentication/test_backends.py
@@ -346,8 +346,11 @@ def test_authentication_getter_existing_user_change_fields(
 
     # One and only one additional update query when a field has changed
     # Note: .save() triggers uniqueness validation queries for unique fields,
-    # adding extra SELECT queries before the UPDATE (e.g., checking unique=True on 'sub')
-    with django_assert_num_queries(3):
+    # adding extra SELECT queries before the UPDATE:
+    #   - unique=True on 'sub'
+    #   - unique=True on 'admin_email'
+    #   - partial unique index 'unique_email_when_sub_is_null'
+    with django_assert_num_queries(5):
         authenticated_user = klass.get_or_create_user(
             access_token="test-token", id_token=None, payload=None
         )
diff --git a/src/backend/core/tests/test_models_users.py b/src/backend/core/tests/test_models_users.py
index edea5bb9..cb23cecd 100644
--- a/src/backend/core/tests/test_models_users.py
+++ b/src/backend/core/tests/test_models_users.py
@@ -44,3 +44,53 @@ def test_models_users_send_mail_main_missing():
         user.email_user("my subject", "my message")
 
     assert str(excinfo.value) == "User has no email address."
+
+
+def test_models_users_email_unique_when_sub_is_null():
+    """Email should be unique among users with no sub (pending users)."""
+    user = factories.UserFactory(sub=None, email="test@example.com")
+    with pytest.raises(
+        ValidationError, match="Constraint “unique_email_when_sub_is_null” is violated."
+    ):
+        factories.UserFactory(sub=None, email=user.email)
+
+
+def test_models_users_email_unique_case_insensitive_when_sub_is_null():
+    """Email uniqueness should be case-insensitive among users with no sub (pending users)."""
+    factories.UserFactory(sub=None, email="Test@example.com")
+    with pytest.raises(
+        ValidationError, match="Constraint “unique_email_when_sub_is_null” is violated."
+    ):
+        factories.UserFactory(sub=None, email="test@example.com")
+
+
+def test_models_users_email_not_unique_when_sub_is_set():
+    """Email uniqueness should not be enforced when users have a sub."""
+    user = factories.UserFactory(sub="sub-1", email="test@example.com")
+    user2 = factories.UserFactory(sub="sub-2", email=user.email)
+    assert user2.email == user.email
+
+
+def test_models_users_email_not_unique_between_sub_null_and_sub_set():
+    """A user with a sub and a pending user (sub=None) can share the same email."""
+    user = factories.UserFactory(sub="sub-1", email="test@example.com")
+    user2 = factories.UserFactory(sub=None, email=user.email)
+    assert user2.email == user.email
+
+
+def test_models_users_email_unique_constraint_allows_multiple_null_emails():
+    """Multiple users with sub=None and email=None should be allowed."""
+    factories.UserFactory(sub=None, email=None)
+    factories.UserFactory(sub=None, email=None)
+
+
+def test_models_users_sub_null_email_null_does_not_prevent_creation():
+    """Multiple pending users (sub=None, email=None) can be created without conflict.
+
+    sub=None is not unique-constrained. email uniqueness is only enforced among
+    sub=None users with a non-null email, so email=None bypasses it (NULL != NULL in SQL).
+    """
+    # Ghost row can still appear from bad code path
+    u1 = factories.UserFactory(sub=None, email=None)
+    u2 = factories.UserFactory(sub=None, email=None)
+    assert u1.pk != u2.pk