Handle the case that the CMS username already exists

api/v3/Cmsuser/Create.php

@@ -37,9 +37,13 @@ function _civicrm_api3_cmsuser_create_spec(&$params) {
  */
 function civicrm_api3_cmsuser_create($params) {
   // Check for existing contact
-  $ufID = CRM_Core_BAO_UFMatch::getUFId($params['contact_id']);
-  if ($ufID) {
-    return civicrm_api3_create_success(['uf_id' => $ufID, 'created' => FALSE], $params);
+  $ufMatchByContactID = \Civi\Api4\UFMatch::get(FALSE)
+    ->addWhere('contact_id', '=', $params['contact_id'])
+    ->execute()
+    ->first();
+
+  if (!empty($ufMatchByContactID)) {
+    return civicrm_api3_create_success(['uf_id' => $ufMatchByContactID['id'], 'created' => FALSE], $params);
   }
 
   // Get email (either from param or from contact record.
@@ -61,6 +65,14 @@ function civicrm_api3_cmsuser_create($params) {
     $params['cms_name'] = $params['email'];
   }
 
+  $ufMatchByName = \Civi\Api4\UFMatch::get(FALSE)
+    ->addWhere('uf_name', '=', $params['cms_name'])
+    ->execute()
+    ->first();
+  if (!empty($ufMatchByName)) {
+    throw new CiviCRM_API3_Exception("Cannot create CMS user for contact ID: {$params['contact_id']}. CMS Username: {$params['cms_name']} already exists.");
+  }
+
   // If no password specified generate a random one
   if (empty($params['cms_pass'])) {
     $params['cms_pass'] = bin2hex(random_bytes(10));