custom-element-define-twice.js

(function() {
  const _customElementsDefine = window.customElements.define;
  window.customElements.define = (name, cl, conf) => {
    if (customElements.get(name)) {
      console.warn(name + 'has been defined twice');
    } else {
      _customElementsDefine.call(window.customElements, name, cl, conf);
    }
  };

})();