diff --git a/backend/endpoints/tests/test_identity.py b/backend/endpoints/tests/test_identity.py
index 5bfbbff26..e66043d34 100644
--- a/backend/endpoints/tests/test_identity.py
+++ b/backend/endpoints/tests/test_identity.py
@@ -131,6 +131,22 @@ def test_add_user_from_unauthorized_user(
         assert response.status_code == expected_status_code
 
 
+def test_add_user_with_existing_username(client, access_token, admin_user):
+    response = client.post(
+        "/api/users",
+        params={
+            "username": admin_user.username,
+            "password": "new_user_password",
+            "role": Role.VIEWER.value,
+        },
+        headers={"Authorization": f"Bearer {access_token}"},
+    )
+    assert response.status_code == HTTPStatus.BAD_REQUEST
+
+    response = response.json()
+    assert response["detail"] == f"Username {admin_user.username} already exists"
+
+
 def test_update_user(client, access_token, editor_user):
     assert editor_user.role == Role.EDITOR
 
diff --git a/backend/endpoints/user.py b/backend/endpoints/user.py
index 113d04387..3bed5391b 100644
--- a/backend/endpoints/user.py
+++ b/backend/endpoints/user.py
@@ -47,7 +47,8 @@ def add_user(request: Request, username: str, password: str, role: str) -> UserS
             detail="Forbidden",
         )
 
-    if username in [user.username for user in db_user_handler.get_users()]:
+    existing_user = db_user_handler.get_user_by_username(username)
+    if existing_user:
         msg = f"Username {username} already exists"
         log.error(msg)
         raise HTTPException(
diff --git a/backend/handler/database/users_handler.py b/backend/handler/database/users_handler.py
index f0a6eafa4..92a467424 100644
--- a/backend/handler/database/users_handler.py
+++ b/backend/handler/database/users_handler.py
@@ -12,11 +12,13 @@ class DBUsersHandler(DBBaseHandler):
         return session.merge(user)
 
     @begin_session
-    def get_user_by_username(self, username: str, session: Session = None):
+    def get_user_by_username(
+        self, username: str, session: Session = None
+    ) -> User | None:
         return session.scalar(select(User).filter_by(username=username).limit(1))
 
     @begin_session
-    def get_user(self, id: int, session: Session = None) -> User:
+    def get_user(self, id: int, session: Session = None) -> User | None:
         return session.get(User, id)
 
     @begin_session