Load jQuery only if has not already been loaded

gistfile1.js

if (typeof jQuery == 'undefined') {
  function getScript(url) {
    var script = document.createElement('script');
    script.src = url;

    var head = document.getElementsByTagName('head')[0],
        done = false;

    // Attach handlers for all browsers
    script.onload = script.onreadystatechange = function() {
      if (!done && (!this.readyState || this.readyState == 'loaded' || this.readyState == 'complete')) {
        done = true;
        script.onload = script.onreadystatechange = null;
        head.removeChild(script);
      }
    }

    head.appendChild(script);
  }

  getScript("absolute_url_to_jquery.js");
}