Index: branches/5.2.x/core/install/english.lang
===================================================================
--- branches/5.2.x/core/install/english.lang
+++ branches/5.2.x/core/install/english.lang
@@ -268,6 +268,7 @@
 			<PHRASE Label="la_error_EmailTemplateBodyMissing" Module="Core" Type="1">RW1haWwgRGVzaWduIFRlbXBsYXRlIHNob3VsZCBjb250YWluIGF0IGxlYXN0ICIkYm9keSIgdGFnIGluIGl0Lg==</PHRASE>
 			<PHRASE Label="la_error_FileNotFound" Module="Core" Type="1">RmlsZSBub3QgZm91bmQ=</PHRASE>
 			<PHRASE Label="la_error_FileTooLarge" Module="Core" Type="1">RmlsZSBpcyB0b28gbGFyZ2U=</PHRASE>
+			<PHRASE Label="la_error_FriendlyUrlIsNotUnique" Module="Core" Type="1">VGhpcyBVUkwgaXMgY29uZmxpY3Rpbmcgd2l0aCBleGlzdGluZyBVUkwgYW5kIGNhbid0IGJlIHVzZWQ=</PHRASE>
 			<PHRASE Label="la_error_GroupNotFound" Module="Core" Type="1">Z3JvdXAgbm90IGZvdW5k</PHRASE>
 			<PHRASE Label="la_error_InvalidFieldName" Module="Core" Type="1">RmllbGQgZG9lc24ndCBleGlzdCBpbiAiJXMiIHVuaXQgY29uZmln</PHRASE>
 			<PHRASE Label="la_error_InvalidFileFormat" Module="Core" Type="1">SW52YWxpZCBGaWxlIEZvcm1hdA==</PHRASE>
Index: branches/5.2.x/core/units/categories/categories_config.php
===================================================================
--- branches/5.2.x/core/units/categories/categories_config.php
+++ branches/5.2.x/core/units/categories/categories_config.php
@@ -402,7 +402,12 @@
 			'default' => NULL
 		),
 		'FormSubmittedTemplate' => Array ('type' => 'string', 'default' => null),
-		'FriendlyURL' => Array ('type' => 'string', 'not_null' => 1, 'default' => ''),
+		'FriendlyURL' => Array (
+			'type' => 'string',
+			'unique' => array(),
+			'error_msgs' => array('unique' => '!la_error_FriendlyUrlIsNotUnique!'),
+			'not_null' => 1, 'default' => '',
+		),
 		'ThemeId' => Array ('type' => 'int', 'not_null' => 1, 'default' => 0),
 
 		'EnablePageCache' => Array (
Index: branches/5.2.x/core/units/categories/categories_event_handler.php
===================================================================
--- branches/5.2.x/core/units/categories/categories_event_handler.php
+++ branches/5.2.x/core/units/categories/categories_event_handler.php
@@ -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