diff --git a/CHANGELOG.md b/CHANGELOG.md
index 28e07ec4..71f5bccc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -29,6 +29,9 @@ and this project adheres to
 - 💄(frontend) update doc versioning ui #463
 - 💄(frontend) update doc summary ui #473
 
+## Fixed
+
+- 🐛(backend) fix create document via s2s if sub unknown but email found #543
 
 ## [1.10.0] - 2024-12-17
 
diff --git a/src/backend/core/api/serializers.py b/src/backend/core/api/serializers.py
index f8dbb22d..e0302042 100644
--- a/src/backend/core/api/serializers.py
+++ b/src/backend/core/api/serializers.py
@@ -264,13 +264,17 @@ class ServerCreateDocumentSerializer(serializers.Serializer):
         """Create the document and associate it with the user or send an invitation."""
         language = validated_data.get("language", settings.LANGUAGE_CODE)
 
-        # Get the user based on the sub (unique identifier)
+        # Get the user on its sub (unique identifier). Default on email if allowed in settings
+        email = validated_data["email"]
+
         try:
-            user = models.User.objects.get(sub=validated_data["sub"])
-        except (models.User.DoesNotExist, KeyError):
-            user = None
-            email = validated_data["email"]
-        else:
+            user = models.User.objects.get_user_by_sub_or_email(
+                validated_data["sub"], email
+            )
+        except models.DuplicateEmailError as err:
+            raise serializers.ValidationError({"email": [err.message]}) from err
+
+        if user:
             email = user.email
             language = user.language or language
 
@@ -279,7 +283,9 @@ class ServerCreateDocumentSerializer(serializers.Serializer):
                 validated_data["content"]
             )
         except ConversionError as err:
-            raise exceptions.APIException(detail="could not convert content") from err
+            raise serializers.ValidationError(
+                {"content": ["Could not convert content"]}
+            ) from err
 
         document = models.Document.objects.create(
             title=validated_data["title"],
@@ -302,7 +308,11 @@ class ServerCreateDocumentSerializer(serializers.Serializer):
                 role=models.RoleChoices.OWNER,
             )
 
-        # Notify the user about the newly created document
+        self._send_email_notification(document, validated_data, email, language)
+        return document
+
+    def _send_email_notification(self, document, validated_data, email, language):
+        """Notify the user about the newly created document."""
         subject = validated_data.get("subject") or _(
             "A new document was created on your behalf!"
         )
@@ -313,8 +323,6 @@ class ServerCreateDocumentSerializer(serializers.Serializer):
         }
         document.send_email(subject, [email], context, language)
 
-        return document
-
     def update(self, instance, validated_data):
         """
         This serializer does not support updates.
diff --git a/src/backend/core/authentication/backends.py b/src/backend/core/authentication/backends.py
index 63932858..f8a7486d 100644
--- a/src/backend/core/authentication/backends.py
+++ b/src/backend/core/authentication/backends.py
@@ -11,7 +11,7 @@ from mozilla_django_oidc.auth import (
     OIDCAuthenticationBackend as MozillaOIDCAuthenticationBackend,
 )
 
-from core.models import User
+from core.models import DuplicateEmailError, User
 
 logger = logging.getLogger(__name__)
 
@@ -98,7 +98,10 @@ class OIDCAuthenticationBackend(MozillaOIDCAuthenticationBackend):
             "short_name": short_name,
         }
 
-        user = self.get_existing_user(sub, email)
+        try:
+            user = User.objects.get_user_by_sub_or_email(sub, email)
+        except DuplicateEmailError as err:
+            raise SuspiciousOperation(err.message) from err
 
         if user:
             if not user.is_active:
@@ -117,16 +120,6 @@ class OIDCAuthenticationBackend(MozillaOIDCAuthenticationBackend):
         )
         return full_name or None
 
-    def get_existing_user(self, sub, email):
-        """Fetch an existing user by sub (or email as a fallback respecting fallback setting."""
-        try:
-            return User.objects.get(sub=sub)
-        except User.DoesNotExist:
-            if email and settings.OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION:
-                return User.objects.filter(email=email).first()
-
-        return None
-
     def update_user_if_needed(self, user, claims):
         """Update user claims if they have changed."""
         has_changed = any(
diff --git a/src/backend/core/models.py b/src/backend/core/models.py
index b7fb8e79..52e66aab 100644
--- a/src/backend/core/models.py
+++ b/src/backend/core/models.py
@@ -1,6 +1,7 @@
 """
 Declare and configure the models for the impress core application
 """
+# pylint: disable=too-many-lines
 
 import hashlib
 import smtplib
@@ -89,6 +90,16 @@ class LinkReachChoices(models.TextChoices):
     PUBLIC = "public", _("Public")  # Even anonymous users can access the document
 
 
+class DuplicateEmailError(Exception):
+    """Raised when an email is already associated with a pre-existing user."""
+
+    def __init__(self, message=None, email=None):
+        """Set message and email to describe the exception."""
+        self.message = message
+        self.email = email
+        super().__init__(self.message)
+
+
 class BaseModel(models.Model):
     """
     Serves as an abstract base model for other models, ensuring that records are validated
@@ -126,6 +137,35 @@ class BaseModel(models.Model):
         super().save(*args, **kwargs)
 
 
+class UserManager(auth_models.UserManager):
+    """Custom manager for User model with additional methods."""
+
+    def get_user_by_sub_or_email(self, sub, email):
+        """Fetch existing user by sub or email."""
+        try:
+            return self.get(sub=sub)
+        except self.model.DoesNotExist as err:
+            if not email:
+                return None
+
+            if settings.OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION:
+                try:
+                    return self.get(email=email)
+                except self.model.DoesNotExist:
+                    pass
+            elif (
+                self.filter(email=email).exists()
+                and not settings.OIDC_ALLOW_DUPLICATE_EMAILS
+            ):
+                raise DuplicateEmailError(
+                    _(
+                        "We couldn't find a user with this sub but the email is already "
+                        "associated with a registered user."
+                    )
+                ) from err
+        return None
+
+
 class User(AbstractBaseUser, BaseModel, auth_models.PermissionsMixin):
     """User model to work with OIDC only authentication."""
 
@@ -192,7 +232,7 @@ class User(AbstractBaseUser, BaseModel, auth_models.PermissionsMixin):
         ),
     )
 
-    objects = auth_models.UserManager()
+    objects = UserManager()
 
     USERNAME_FIELD = "admin_email"
     REQUIRED_FIELDS = []
@@ -939,7 +979,10 @@ class Invitation(BaseModel):
         super().clean()
 
         # Check if an identity already exists for the provided email
-        if User.objects.filter(email=self.email).exists():
+        if (
+            User.objects.filter(email=self.email).exists()
+            and not settings.OIDC_ALLOW_DUPLICATE_EMAILS
+        ):
             raise exceptions.ValidationError(
                 {"email": _("This email is already associated to a registered user.")}
             )
diff --git a/src/backend/core/tests/authentication/test_backends.py b/src/backend/core/tests/authentication/test_backends.py
index c73099a6..8bd47cab 100644
--- a/src/backend/core/tests/authentication/test_backends.py
+++ b/src/backend/core/tests/authentication/test_backends.py
@@ -1,5 +1,6 @@
 """Unit tests for the Authentication Backends."""
 
+import random
 import re
 from logging import Logger
 from unittest import mock
@@ -64,7 +65,33 @@ def test_authentication_getter_existing_user_via_email(
     assert user == db_user
 
 
-def test_authentication_getter_existing_user_no_fallback_to_email(
+def test_authentication_getter_email_none(monkeypatch):
+    """
+    If no user is found with the sub and no email is provided, a new user should be created.
+    """
+
+    klass = OIDCAuthenticationBackend()
+    db_user = UserFactory(email=None)
+
+    def get_userinfo_mocked(*args):
+        user_info = {"sub": "123"}
+        if random.choice([True, False]):
+            user_info["email"] = None
+        return user_info
+
+    monkeypatch.setattr(OIDCAuthenticationBackend, "get_userinfo", get_userinfo_mocked)
+
+    user = klass.get_or_create_user(
+        access_token="test-token", id_token=None, payload=None
+    )
+
+    # Since the sub and email didn't match, it should create a new user
+    assert models.User.objects.count() == 2
+    assert user != db_user
+    assert user.sub == "123"
+
+
+def test_authentication_getter_existing_user_no_fallback_to_email_allow_duplicate(
     settings, monkeypatch
 ):
     """
@@ -77,6 +104,7 @@ def test_authentication_getter_existing_user_no_fallback_to_email(
 
     # Set the setting to False
     settings.OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION = False
+    settings.OIDC_ALLOW_DUPLICATE_EMAILS = True
 
     def get_userinfo_mocked(*args):
         return {"sub": "123", "email": db_user.email}
@@ -93,6 +121,39 @@ def test_authentication_getter_existing_user_no_fallback_to_email(
     assert user.sub == "123"
 
 
+def test_authentication_getter_existing_user_no_fallback_to_email_no_duplicate(
+    settings, monkeypatch
+):
+    """
+    When the "OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION" setting is set to False,
+    the system should not match users by email, even if the email matches.
+    """
+
+    klass = OIDCAuthenticationBackend()
+    db_user = UserFactory()
+
+    # Set the setting to False
+    settings.OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION = False
+    settings.OIDC_ALLOW_DUPLICATE_EMAILS = False
+
+    def get_userinfo_mocked(*args):
+        return {"sub": "123", "email": db_user.email}
+
+    monkeypatch.setattr(OIDCAuthenticationBackend, "get_userinfo", get_userinfo_mocked)
+
+    with pytest.raises(
+        SuspiciousOperation,
+        match=(
+            "We couldn't find a user with this sub but the email is already associated "
+            "with a registered user."
+        ),
+    ):
+        klass.get_or_create_user(access_token="test-token", id_token=None, payload=None)
+
+    # Since the sub doesn't match, it should not create a new user
+    assert models.User.objects.count() == 1
+
+
 def test_authentication_getter_existing_user_with_email(
     django_assert_num_queries, monkeypatch
 ):
diff --git a/src/backend/core/tests/documents/test_api_documents_create_for_owner.py b/src/backend/core/tests/documents/test_api_documents_create_for_owner.py
index 4c66a286..084aaec1 100644
--- a/src/backend/core/tests/documents/test_api_documents_create_for_owner.py
+++ b/src/backend/core/tests/documents/test_api_documents_create_for_owner.py
@@ -13,6 +13,7 @@ import pytest
 from rest_framework.test import APIClient
 
 from core import factories
+from core.api.serializers import ServerCreateDocumentSerializer
 from core.models import Document, Invitation, User
 from core.services.converter_services import ConversionError, YdocConverter
 
@@ -20,7 +21,7 @@ pytestmark = pytest.mark.django_db
 
 
 @pytest.fixture
-def mock_convert_markdown():
+def mock_convert_md():
     """Mock YdocConverter.convert_markdown to return a converted content."""
     with patch.object(
         YdocConverter,
@@ -169,8 +170,11 @@ def test_api_documents_create_for_owner_invalid_sub():
 
 
 @override_settings(SERVER_TO_SERVER_API_TOKENS=["DummyToken"])
-def test_api_documents_create_for_owner_existing(mock_convert_markdown):
-    """It should be possible to create a document on behalf of a pre-existing user."""
+def test_api_documents_create_for_owner_existing(mock_convert_md):
+    """
+    It should be possible to create a document on behalf of a pre-existing user
+    by passing their sub and email.
+    """
     user = factories.UserFactory(language="en-us")
 
     data = {
@@ -189,7 +193,7 @@ def test_api_documents_create_for_owner_existing(mock_convert_markdown):
 
     assert response.status_code == 201
 
-    mock_convert_markdown.assert_called_once_with("Document content")
+    mock_convert_md.assert_called_once_with("Document content")
 
     document = Document.objects.get()
     assert response.json() == {"id": str(document.id)}
@@ -213,10 +217,10 @@ def test_api_documents_create_for_owner_existing(mock_convert_markdown):
 
 
 @override_settings(SERVER_TO_SERVER_API_TOKENS=["DummyToken"])
-def test_api_documents_create_for_owner_new_user(mock_convert_markdown):
+def test_api_documents_create_for_owner_new_user(mock_convert_md):
     """
     It should be possible to create a document on behalf of new users by
-    passing only their email address.
+    passing their unknown sub and email address.
     """
     data = {
         "title": "My Document",
@@ -234,7 +238,7 @@ def test_api_documents_create_for_owner_new_user(mock_convert_markdown):
 
     assert response.status_code == 201
 
-    mock_convert_markdown.assert_called_once_with("Document content")
+    mock_convert_md.assert_called_once_with("Document content")
 
     document = Document.objects.get()
     assert response.json() == {"id": str(document.id)}
@@ -264,8 +268,190 @@ def test_api_documents_create_for_owner_new_user(mock_convert_markdown):
     assert document.creator == user
 
 
+@override_settings(
+    SERVER_TO_SERVER_API_TOKENS=["DummyToken"],
+    OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION=True,
+)
+def test_api_documents_create_for_owner_existing_user_email_no_sub_with_fallback(
+    mock_convert_md,
+):
+    """
+    It should be possible to create a document on behalf of a pre-existing user for
+    who the sub was not found if the settings allow it. This edge case should not
+    happen in a healthy OIDC federation but can be usefull if an OIDC provider modifies
+    users sub on each login for example...
+    """
+    user = factories.UserFactory(language="en-us")
+
+    data = {
+        "title": "My Document",
+        "content": "Document content",
+        "sub": "123",
+        "email": user.email,
+    }
+
+    response = APIClient().post(
+        "/api/v1.0/documents/create-for-owner/",
+        data,
+        format="json",
+        HTTP_AUTHORIZATION="Bearer DummyToken",
+    )
+
+    assert response.status_code == 201
+
+    mock_convert_md.assert_called_once_with("Document content")
+
+    document = Document.objects.get()
+    assert response.json() == {"id": str(document.id)}
+
+    assert document.title == "My Document"
+    assert document.content == "Converted document content"
+    assert document.creator == user
+    assert document.accesses.filter(user=user, role="owner").exists()
+
+    assert Invitation.objects.exists() is False
+
+    assert len(mail.outbox) == 1
+    email = mail.outbox[0]
+    assert email.to == [user.email]
+    assert email.subject == "A new document was created on your behalf!"
+    email_content = " ".join(email.body.split())
+    assert "A new document was created on your behalf!" in email_content
+    assert (
+        "You have been granted ownership of a new document: My Document"
+    ) in email_content
+
+
+@override_settings(
+    SERVER_TO_SERVER_API_TOKENS=["DummyToken"],
+    OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION=False,
+    OIDC_ALLOW_DUPLICATE_EMAILS=False,
+)
+def test_api_documents_create_for_owner_existing_user_email_no_sub_no_fallback(
+    mock_convert_md,
+):
+    """
+    When a user does not match an existing sub and fallback to matching on email is
+    not allowed in settings, it should raise an error if the email is already used by
+    a registered user and duplicate emails are not allowed.
+    """
+    user = factories.UserFactory()
+
+    data = {
+        "title": "My Document",
+        "content": "Document content",
+        "sub": "123",
+        "email": user.email,
+    }
+
+    response = APIClient().post(
+        "/api/v1.0/documents/create-for-owner/",
+        data,
+        format="json",
+        HTTP_AUTHORIZATION="Bearer DummyToken",
+    )
+    assert response.status_code == 400
+    assert response.json() == {
+        "email": [
+            (
+                "We couldn't find a user with this sub but the email is already "
+                "associated with a registered user."
+            )
+        ]
+    }
+    assert mock_convert_md.called is False
+    assert Document.objects.exists() is False
+    assert Invitation.objects.exists() is False
+    assert len(mail.outbox) == 0
+
+
+@override_settings(
+    SERVER_TO_SERVER_API_TOKENS=["DummyToken"],
+    OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION=False,
+    OIDC_ALLOW_DUPLICATE_EMAILS=True,
+)
+def test_api_documents_create_for_owner_new_user_no_sub_no_fallback_allow_duplicate(
+    mock_convert_md,
+):
+    """
+    When a user does not match an existing sub and fallback to matching on email is
+    not allowed in settings, it should be possible to create a new user with the same
+    email as an existing user if the settings allow it (identification is still done
+    via the sub in this case).
+    """
+    user = factories.UserFactory()
+
+    data = {
+        "title": "My Document",
+        "content": "Document content",
+        "sub": "123",
+        "email": user.email,
+    }
+
+    response = APIClient().post(
+        "/api/v1.0/documents/create-for-owner/",
+        data,
+        format="json",
+        HTTP_AUTHORIZATION="Bearer DummyToken",
+    )
+    assert response.status_code == 201
+    mock_convert_md.assert_called_once_with("Document content")
+
+    document = Document.objects.get()
+    assert response.json() == {"id": str(document.id)}
+
+    assert document.title == "My Document"
+    assert document.content == "Converted document content"
+    assert document.creator is None
+    assert document.accesses.exists() is False
+
+    invitation = Invitation.objects.get()
+    assert invitation.email == user.email
+    assert invitation.role == "owner"
+
+    assert len(mail.outbox) == 1
+    email = mail.outbox[0]
+    assert email.to == [user.email]
+    assert email.subject == "A new document was created on your behalf!"
+    email_content = " ".join(email.body.split())
+    assert "A new document was created on your behalf!" in email_content
+    assert (
+        "You have been granted ownership of a new document: My Document"
+    ) in email_content
+
+    # The creator field on the document should be set when the user is created
+    user = User.objects.create(email=user.email, password="!")
+    document.refresh_from_db()
+    assert document.creator == user
+
+
+@patch.object(ServerCreateDocumentSerializer, "_send_email_notification")
+@override_settings(SERVER_TO_SERVER_API_TOKENS=["DummyToken"], LANGUAGE_CODE="de-de")
+def test_api_documents_create_for_owner_with_default_language(
+    mock_send, mock_convert_md
+):
+    """The default language from settings should apply by default."""
+    data = {
+        "title": "My Document",
+        "content": "Document content",
+        "sub": "123",
+        "email": "john.doe@example.com",
+    }
+
+    response = APIClient().post(
+        "/api/v1.0/documents/create-for-owner/",
+        data,
+        format="json",
+        HTTP_AUTHORIZATION="Bearer DummyToken",
+    )
+    assert response.status_code == 201
+
+    mock_convert_md.assert_called_once_with("Document content")
+    assert mock_send.call_args[0][3] == "de-de"
+
+
 @override_settings(SERVER_TO_SERVER_API_TOKENS=["DummyToken"])
-def test_api_documents_create_for_owner_with_custom_language(mock_convert_markdown):
+def test_api_documents_create_for_owner_with_custom_language(mock_convert_md):
     """
     Test creating a document with a specific language.
     Useful if the remote server knows the user's language.
@@ -287,7 +473,7 @@ def test_api_documents_create_for_owner_with_custom_language(mock_convert_markdo
 
     assert response.status_code == 201
 
-    mock_convert_markdown.assert_called_once_with("Document content")
+    mock_convert_md.assert_called_once_with("Document content")
 
     assert len(mail.outbox) == 1
     email = mail.outbox[0]
@@ -302,7 +488,7 @@ def test_api_documents_create_for_owner_with_custom_language(mock_convert_markdo
 
 @override_settings(SERVER_TO_SERVER_API_TOKENS=["DummyToken"])
 def test_api_documents_create_for_owner_with_custom_subject_and_message(
-    mock_convert_markdown,
+    mock_convert_md,
 ):
     """It should be possible to customize the subject and message of the invitation email."""
     data = {
@@ -323,7 +509,7 @@ def test_api_documents_create_for_owner_with_custom_subject_and_message(
 
     assert response.status_code == 201
 
-    mock_convert_markdown.assert_called_once_with("Document content")
+    mock_convert_md.assert_called_once_with("Document content")
 
     assert len(mail.outbox) == 1
     email = mail.outbox[0]
@@ -336,11 +522,11 @@ def test_api_documents_create_for_owner_with_custom_subject_and_message(
 
 @override_settings(SERVER_TO_SERVER_API_TOKENS=["DummyToken"])
 def test_api_documents_create_for_owner_with_converter_exception(
-    mock_convert_markdown,
+    mock_convert_md,
 ):
-    """It should be possible to customize the subject and message of the invitation email."""
+    """In case of converter error, a 400 error should be raised."""
 
-    mock_convert_markdown.side_effect = ConversionError("Conversion failed")
+    mock_convert_md.side_effect = ConversionError("Conversion failed")
 
     data = {
         "title": "My Document",
@@ -357,8 +543,33 @@ def test_api_documents_create_for_owner_with_converter_exception(
         format="json",
         HTTP_AUTHORIZATION="Bearer DummyToken",
     )
+    mock_convert_md.assert_called_once_with("Document content")
 
-    mock_convert_markdown.assert_called_once_with("Document content")
+    assert response.status_code == 400
+    assert response.json() == {"content": ["Could not convert content"]}
 
-    assert response.status_code == 500
-    assert response.json() == {"detail": "could not convert content"}
+
+@override_settings(SERVER_TO_SERVER_API_TOKENS=["DummyToken"])
+def test_api_documents_create_for_owner_with_empty_content():
+    """The content should not be empty or a 400 error should be raised."""
+
+    data = {
+        "title": "My Document",
+        "content": "  ",
+        "sub": "123",
+        "email": "john.doe@example.com",
+    }
+
+    response = APIClient().post(
+        "/api/v1.0/documents/create-for-owner/",
+        data,
+        format="json",
+        HTTP_AUTHORIZATION="Bearer DummyToken",
+    )
+
+    assert response.status_code == 400
+    assert response.json() == {
+        "content": [
+            "This field may not be blank.",
+        ],
+    }
diff --git a/src/backend/core/tests/test_settings.py b/src/backend/core/tests/test_settings.py
new file mode 100644
index 00000000..44c7c8f7
--- /dev/null
+++ b/src/backend/core/tests/test_settings.py
@@ -0,0 +1,30 @@
+"""
+Unit tests for the User model
+"""
+
+import pytest
+
+from impress.settings import Base
+
+
+def test_invalid_settings_oidc_email_configuration():
+    """
+    The OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION and OIDC_ALLOW_DUPLICATE_EMAILS settings
+    should not be both set to True simultaneously.
+    """
+
+    class TestSettings(Base):
+        """Fake test settings."""
+
+        OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION = True
+        OIDC_ALLOW_DUPLICATE_EMAILS = True
+
+    # The validation is performed during post_setup
+    with pytest.raises(ValueError) as excinfo:
+        TestSettings().post_setup()
+
+    # Check the exception message
+    assert str(excinfo.value) == (
+        "Both OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION and "
+        "OIDC_ALLOW_DUPLICATE_EMAILS cannot be set to True simultaneously. "
+    )
diff --git a/src/backend/impress/settings.py b/src/backend/impress/settings.py
index 41557bc2..495ec4bc 100755
--- a/src/backend/impress/settings.py
+++ b/src/backend/impress/settings.py
@@ -474,6 +474,15 @@ class Base(Configuration):
         environ_prefix=None,
     )
 
+    # WARNING: Enabling this setting allows multiple user accounts to share the same email
+    # address. This may cause security issues and is not recommended for production use when
+    # email is activated as fallback for identification (see previous setting).
+    OIDC_ALLOW_DUPLICATE_EMAILS = values.BooleanValue(
+        default=False,
+        environ_name="OIDC_ALLOW_DUPLICATE_EMAILS",
+        environ_prefix=None,
+    )
+
     USER_OIDC_ESSENTIAL_CLAIMS = values.ListValue(
         default=[], environ_name="USER_OIDC_ESSENTIAL_CLAIMS", environ_prefix=None
     )
@@ -625,6 +634,15 @@ class Base(Configuration):
             with sentry_sdk.configure_scope() as scope:
                 scope.set_extra("application", "backend")
 
+        if (
+            cls.OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION
+            and cls.OIDC_ALLOW_DUPLICATE_EMAILS
+        ):
+            raise ValueError(
+                "Both OIDC_FALLBACK_TO_EMAIL_FOR_IDENTIFICATION and "
+                "OIDC_ALLOW_DUPLICATE_EMAILS cannot be set to True simultaneously. "
+            )
+
 
 class Build(Base):
     """Settings used when the application is built.