Added a check for unique access token constraint violation

src/Exception/UniqueTokenIdentifierConstraintViolationException.php

@@ -0,0 +1,21 @@
+<?php
+/**
+ * @author      Ivan Kurnosov <zerkms@zerkms.com>
+ * @copyright   Copyright (c) Alex Bilbie
+ * @license     http://mit-license.org/
+ *
+ * @link        https://github.com/thephpleague/oauth2-server
+ */
+
+namespace League\OAuth2\Server\Exception;
+
+
+class UniqueTokenIdentifierConstraintViolationException extends OAuthServerException
+{
+    public static function create()
+    {
+        $errorMessage = 'Could not create unique access token identifier';
+
+        return new static($errorMessage, 100, 'access_token_duplicate', 500);
+    }
+}