diff --git a/amanuensis/backend/user.py b/amanuensis/backend/user.py
index 9a28e67..98c4c8b 100644
--- a/amanuensis/backend/user.py
+++ b/amanuensis/backend/user.py
@@ -33,6 +33,10 @@ def create_user(
     if not display_name.strip():
         display_name = username
 
+    # Query the db to make sure the username isn't taken
+    if db.session.query(User.username == username).count() > 0:
+        raise ArgumentError('Username is already taken')
+
     new_user = User(
         username=username,
         password=password,
diff --git a/tests/test_db.py b/tests/test_db.py
index 9e551b0..eb05f44 100644
--- a/tests/test_db.py
+++ b/tests/test_db.py
@@ -46,3 +46,6 @@ def test_create_user(db):
     assert new_user
     assert new_user.id is not None
     assert new_user.created is not None
+
+    with pytest.raises(ArgumentError):
+        duplicate = userq.create_user(db, **kwargs)