Index: wp-includes/widgets.php
===================================================================
--- wp-includes/widgets.php	(revision 13697)
+++ wp-includes/widgets.php	(working copy)
@@ -490,7 +490,11 @@
 		}
 
 		if (isset($args['id'])) {
-			$_args['id'] = $args['id'];
+			if ( $number > 1 ){
+		    $_args['id'] = sprintf($args['id'] . '-%d', $i);
+		  } else {
+		    $_args['id'] = $args['id'];
+		  }
 		} else {
 			$n = count($wp_registered_sidebars);
 			do {