Added part of backend but problems with Joomla code compatibility found.

Somehow the password must be handled specially (also because of the hashing) which causes problems at the moment.

src/admin/models/rules/clubusername.php

@@ -0,0 +1,41 @@
+<?php
+
+use Joomla\CMS\Factory;
+use Joomla\CMS\Form\Form;
+use Joomla\CMS\Form\FormRule;
+use Joomla\Registry\Registry;
+
+// No direct access.
+defined('_JEXEC') or die;
+
+class JFormRuleClubUserName extends FormRule
+{
+    
+    public function test(\SimpleXMLElement $element, $value, $group = null, Registry $input = null, Form $form = null)
+    {
+        // Get the database object and a new query object.
+        $db = Factory::getDbo();
+        $query = $db->getQuery(true);
+        
+        // Build the query.
+        $query->select('COUNT(*)')
+        ->from('#__club_users')
+        ->where('user = ' . $db->quote($value));
+        
+        // Get the extra field check attribute.
+        $userId = $input->get('id');
+        $query->where($db->quoteName('id') . ' <> ' . (int) $userId);
+        
+        // Set and query the database.
+        $db->setQuery($query);
+        $duplicate = (bool) $db->loadResult();
+        
+        if ($duplicate)
+        {
+            return false;
+        }
+        
+        return true;
+    }
+    
+}