diff --git a/src/Controller/AddressBookAdminController.php b/src/Controller/AddressBookAdminController.php
index b773e3c..d8014c9 100644
--- a/src/Controller/AddressBookAdminController.php
+++ b/src/Controller/AddressBookAdminController.php
@@ -104,7 +104,11 @@ class AddressBookAdminController
     {
         $this->adminCheck();
 
-        // TODO currently breaks on inserting a duplicate nick
+        $nick = $_POST['nick'];
+
+        if ($this->userRepository->findByNick(nick: $nick)) {
+            die("User: $nick already exists");
+        }
         if (!empty($_POST)) {
             $isAdmin = empty($_POST['is_admin']) ? 0 : 1;
             $user = new User(nick: $_POST['nick'], newPassword: $_POST['new_password'], first: $_POST['first'], last: $_POST['last'], isAdmin: $isAdmin);
@@ -147,8 +151,10 @@ class AddressBookAdminController
                 die("Error deleting user");
             }
         } else {
-            // TODO use 404
-            die("Nick: $nick not found");
+            $this->template->render(templateName: 'status/404.html.php', vars: [
+                'user'   => $this->user,
+                'router' => $this->router
+            ]);
         }
     }