Find and mark elements with on page which have duplicate IDs

duplicate-id-finder.js

javascript: void ((function() {
  const ids = document.querySelectorAll('[id]');
  Array.from(ids).forEach(el => {
    const id = document.querySelectorAll(`[id="${el.id}"]`);
    if (id.length > 1 && id[0] === el) {
      console.log('Duplicate id ' + el.id);
      Array.from(id).forEach(fel => {
        fel.style.outline = '5px solid red';
      });
      alert('duplicate found');
    }
  });
})());