Iron form with polymer 2.0

I'm having a bug, I replicate the bug in jsbin: https://jsbin.com/micinalacu/1/edit?html,console,output

The iron form, when submit the serialize method return always undefined, and it's called two times.

 <dom-module id="my-form">
  <template>

    <iron-form id="myForm">
      <form method="get" action="cenfdsas">
        <input type="text" name="cenas">
        <button on-click="cenas">Submit</button>
      </form>
    </iron-form>

  </template>

  <script>
   class MyForm extends Polymer.Element {

      static get is() {
        return 'my-form';
      }

      connectedCallback() {
        super.connectedCallback();
        const form = this.$.myForm;
        form.addEventListener('iron-form-presubmit', function (event) {
          event.preventDefault();
          console.log("here")
          console.log(form.serialize());
        });
      }

      cenas() {
        this.$.myForm.submit();
      }

    }

    window.customElements.define(MyForm.is, MyForm);

  </script>
</dom-module>

Update

Polymer team needed to change the name of the method to serializeForm, because they had a bug. Source: https://github.com/PolymerElements/iron-form/issues/174

But I continue with the issue that the submit event It's called two times Bug --> https://jsbin.com/koyelafeze/1/edit?html,console,output

According to the documentation, the use of a standard <button> element in the form will submit it automatically.

You should then use a <paper-button> as suggested in the link, or comment the content of your cenas() method.

JS Bin example