Index: wp-includes/meta.php
===================================================================
--- wp-includes/meta.php	(revisão 15734)
+++ wp-includes/meta.php	(cópia de trabalho)
@@ -52,7 +52,12 @@
 
 	$_meta_value = $meta_value;
 	$meta_value = maybe_serialize( stripslashes_deep($meta_value) );
-
+    
+    if ( $wpdb->get_var( $wpdb->prepare(
+		"SELECT COUNT(*) FROM $table WHERE meta_key = %s AND $column = %d AND meta_value = %s",
+		$meta_key, $object_id, $meta_value ) ) )
+		return false;
+    
 	$wpdb->insert( $table, array(
 		$column => $object_id,
 		'meta_key' => $meta_key,