Index: t3lib/class.t3lib_tstemplate.php
===================================================================
--- t3lib/class.t3lib_tstemplate.php	(revision 147844)
+++ t3lib/class.t3lib_tstemplate.php	(working copy)
@@ -404,12 +404,25 @@
 						)
 					);
 				} else {
-					$dbFields = array(
-						'content' => serialize($cc),
-						'tstamp' => $GLOBALS['EXEC_TIME']
-					);
-					$GLOBALS['TYPO3_DB']->exec_UPDATEquery('cache_pagesection', 'page_id=' . intval($GLOBALS['TSFE']->id) . ' AND mpvar_hash=' . $mpvarHash, $dbFields);
-					if ($GLOBALS['TYPO3_DB']->sql_affected_rows() == 0) {
+					/**
+					 * update DB-record (if exists) or update it
+					 * 
+					 * DON'T use this logic:
+					 * 1. Update DB-record
+					 * 2. If 0 rows were affected: Insert DB-record
+					 * 
+					 * This can produce the SQL-error "Duplicate entry '[pageId]-[mpVarHash]' for key 'PRIMARY'", if 0 rows where affected
+					 * because the DB-record is up-to-date (e.g. because a concurrent request has changed the DB-record already)
+					 */
+					$dbFields = array();
+					$dbFields['content'] = serialize($cc);
+					$dbFields['tstamp'] = $GLOBALS['EXEC_TIME'];
+
+					$sqlWhere = 'page_id=' . intval($GLOBALS['TSFE']->id) . ' AND mpvar_hash=' . $mpvarHash;
+					$count = $GLOBALS['TYPO3_DB']->exec_SELECTcountRows('tstamp', 'cache_pagesection', $sqlWhere);
+					if($count > 0) {
+						$GLOBALS['TYPO3_DB']->exec_UPDATEquery('cache_pagesection', $sqlWhere, $dbFields);
+					} else {
 						$dbFields['page_id'] = intval($GLOBALS['TSFE']->id);
 						$dbFields['mpvar_hash'] = $mpvarHash;
 						$GLOBALS['TYPO3_DB']->exec_INSERTquery('cache_pagesection', $dbFields);