Simple way to include a css or js on the fly only once.

include_once.js

/**
 * to includes script/css files only if there are not loaded yet.
 * 
 * @param url url of the resource
 * @param type js or css (empty default is js)
 */
function include_once(url,type)
{
	var tagname;
	var urlattr;
	
	if( type != "css")
	{
		tagname = "script";
		urlattr = "src";
	} else {
		tagname = "link";
		urlattr = "href";
	}
	
	var domelements = document.getElementsByTagName(tagname);
	// if the url is relative the dom element contains the absolute url after injection
	// so we check only if the urls end the same way
	if( domelements ) 
		for( var i=0; i<domelements.length; i++ ) 
			if( domelements[i][urlattr].match(url+"$") == url )
				return;
			
	var newdom = document.createElement(tagname);
	newdom[urlattr] = url;
	
	if( type != "css" )
		newdom.type = 'text/javascript';
	else
		newdom.rel = 'stylesheet';
	
	// add to head or body
	(document.getElementsByTagName('head')[0] || document.body).appendChild(newdom);
}