avoid sending headers twice

header.inc.php

@@ -2,6 +2,8 @@
 /* $Id$ */
 // vim: expandtab sw=4 ts=4 sts=4:
 
+if (empty($GLOBALS['is_header_sent'])) {
+
     /**
      * Gets a core script and starts output buffering work
      */
@@ -151,5 +153,6 @@ echo "\n";
      * Sets a variable to remember headers have been sent
      */
     $GLOBALS['is_header_sent'] = TRUE;
+}
 
 ?>