Index: wp-includes/comment.php
===================================================================
--- wp-includes/comment.php	(revision 5486)
+++ wp-includes/comment.php	(working copy)
@@ -730,7 +730,7 @@
 			// when set to true, this outputs debug messages by itself
 			$client->debug = false;
 
-			if ( $client->query('pingback.ping', $pagelinkedfrom, $pagelinkedto ) )
+			if ( $client->query('pingback.ping', $pagelinkedfrom, $pagelinkedto) || ( isset($client->error->code) && 48 == $client->error->code ) ) // Already registered
 				add_ping( $post_ID, $pagelinkedto );
 		}
 	}
@@ -793,4 +793,4 @@
 		$client->query('weblogUpdates.ping', get_option('blogname'), $home);
 }
 
-?>
\ No newline at end of file
+?>