correctly error on existing user creation

server.py

@@ -10,6 +10,7 @@ from sqlalchemy import Column, Integer, String, Boolean, or_, and_
 from sqlalchemy.orm import sessionmaker
 from sqlalchemy.exc import IntegrityError
 from sqlalchemy.sql import func
+import sqlalchemy
 
 from flask_sqlalchemy import SQLAlchemy
 
@@ -91,11 +92,17 @@ def createUser(webform):
     if username in FORBIDDEN_USERNAMES:
         return "Error: Username {} is forbidden because it is a special user.".format(username)
 
-    subprocess.run(["/usr/bin/sudo", "./scripts/create_user.sh", cryptPass, username])
-
     # track added users to prevent deletion of other users and listing #
-    db.session.add(FTPUser(username=webform['username']))
-    db.session.commit()
+    try:
+        db.session.add(FTPUser(username=webform['username']))
+        db.session.commit()
+    except sqlalchemy.exc.IntegrityError as e:
+        if "UNIQUE" in str(e):
+            return "Error: User with this name already exists"
+        else:
+            return str(e)
+
+    subprocess.run(["/usr/bin/sudo", "./scripts/create_user.sh", cryptPass, username])
 
     return None