diff --git a/src/applications/people/xaction/PhabricatorUserUsernameTransaction.php b/src/applications/people/xaction/PhabricatorUserUsernameTransaction.php
--- a/src/applications/people/xaction/PhabricatorUserUsernameTransaction.php
+++ b/src/applications/people/xaction/PhabricatorUserUsernameTransaction.php
@@ -70,7 +70,11 @@
           pht('You must be an administrator to rename users.'));
       }
 
-      if (!strlen($new)) {
+      if ($this->generateOldValue($object) === $new) {
+        $errors[] = $this->newInvalidError(
+          pht('New username cannot be the old username.'),
+          $xaction);
+      } else if (!strlen($new)) {
         $errors[] = $this->newInvalidError(
           pht('New username is required.'),
           $xaction);