Unless you check something before attempting to include it, there will always be overhead.
PHP Code:
if (!defined('THISSCRIPT_H'))
{
define('THISSCRIPT_H', true);
// the rest of my code.
}
// included with:
if (!defined('THISSCRIPT_H'))
{
require 'thatscript.php';
}
It won't get any better, especially since php includes off of the executing script, not the script including it. Hence why simply using _once and including it everywhere its needed is better.