Version

3.8.0

Reproduction link

https://codesandbox.io/s/vue-template-p5kdn?fontsize=14

Steps to reproduce

Install @vue/cli;
Create a project;
In main.js change mount to:

new Vue({
  el: "#app", // because this id exists twice
  components: { App },
  template: "<App/>"
});

watch app serve fail because #app exists in App.vue and index.html.

What is expected?

App to mount

What is actually happening?

App is failing to mount due to duplicate id

While helping someone implement ant-design package with vue cli I noticed in App.vue its root element is a div with the id of "app".
Then in public/index.html there is also a div with id "app".

The id should be removed from App.vue