Looking for a function that checks if script was loaded before

Hi,

I am trying to compress all my javascript files and output it as one file.

I am using this code:

<?php
header('Content-Type: text/javascript');

function my_obstart() {
	$encode = getenv('HTTP_ACCEPT_ENCODING');
	if (strstr($encode, 'gzip')) {
		ob_start('ob_gzhandler');
	}
	else {
		ob_start();
	}
}
my_obstart();

require_once 'jquery.js';
require_once 'main.js';
require_once 'boostrap.js';
require_once 'lesscss.js';

ob_end_flush();

In my code above you see that I am using required_once instead of
wp_register_script(‘myscript’, get_template_directory_uri() . ‘/myscript.js’, false, null, true);
wp_enqueue_script(‘myscript’);
Obviously I can not use this function but yet I need to know if the script has been included by any other plugin. So I need a function that gives me either true or false before I use require_once.

I would appreciate any help!