Index: branches/5.2.x/core/units/categories/categories_event_handler.php
===================================================================
diff -u -N -r16563 -r16591
--- branches/5.2.x/core/units/categories/categories_event_handler.php	(.../categories_event_handler.php)	(revision 16563)
+++ branches/5.2.x/core/units/categories/categories_event_handler.php	(.../categories_event_handler.php)	(revision 16591)
@@ -1,6 +1,6 @@
 <?php
 /**
-* @version	$Id: categories_event_handler.php 16563 2017-07-16 15:12:46Z alex $
+* @version	$Id: categories_event_handler.php 16591 2017-07-19 07:50:28Z alex $
 * @package	In-Portal
 * @copyright	Copyright (C) 1997 - 2009 Intechnic. All rights reserved.
 * @license      GNU/GPL
@@ -1369,6 +1369,41 @@
 		}
 
 		/**
+		 * Validates category data.
+		 *
+		 * @param kEvent $event Event.
+		 *
+		 * @return void
+		 */
+		protected function OnBeforeItemValidate(kEvent $event)
+		{
+			parent::OnBeforeItemValidate($event);
+
+			/** @var kDBItem $object */
+			$object = $event->getObject();
+
+			$friendly_url = $object->GetDBField('FriendlyURL');
+
+			if ( strlen($friendly_url) && $friendly_url != $object->GetOriginalField('FriendlyURL') ) {
+				$sql = 'SELECT CategoryId
+						FROM %s
+						WHERE NamedParentPath = ' . $this->Conn->qstr('Content/' . $friendly_url);
+				$duplicate_id = $this->Conn->GetOne(sprintf($sql, $object->TableName));
+
+				if ( $duplicate_id === false && $object->IsTempTable() ) {
+					$duplicate_id = $this->Conn->GetOne(sprintf(
+						$sql,
+						$this->Application->GetLiveName($object->TableName)
+					));
+				}
+
+				if ( $duplicate_id !== false ) {
+					$object->SetError('FriendlyURL', 'unique');
+				}
+			}
+		}
+
+		/**
 		 * Sets correct status for new categories created on front-end
 		 *
 		 * @param kEvent $event