This function will check if there is a script tag with the external file required. If the tag is allready in de document will skip the document write.

function loadScript(src) {
  for(var i = 0; i < scripts.length; i ++) {
    if(scripts[i].src == src) {
      return;
    }
  }
  var script = document.createElement('script');
  script.setAttribute('src', src);
  document.getElementsByTagName('head')[0].appendChild(script);
}

Call this function to insert the script to the document.

loadScript("https://cdnjs.cloudflare.com/ajax/libs/webcomponentsjs/1.0.3/webcomponents-loader.js");