JSFiddle - React, Tailwind, and code Playground

HTML

<p id="dupid">paragraph 1</p>
<p id="dupid">paragraph 2</p>
<p><span id="dupid2">span1</span> <span id="dupid2">span2</span></p>
<p id="id">paragraph 2</p>
<p>dupid count: <span id="dupidcount"></span></p>
<p>all duplicate ids: <span id="dupidlist"></span></p>

JavaScript

function checkDupId(){
	var dupIdCount = document.querySelectorAll('[id="dupid"]').length;
  document.querySelector('#dupidcount').textContent = dupIdCount;
}

checkDupId();

function checkAllIds() {
	var counts = {};
  var duplicates = new Set();
  function addCount(id) {
  	if (counts[id]) {
    	counts[id] += 1;
      duplicates.add(id);
    } else {
    	counts[id] = 1;
    }
  }
	Array.from(document.querySelectorAll("[id]")).forEach(function(elem) {
  	addCount(elem.id);
  })
  return Array.from(duplicates);
}

var dupes = checkAllIds();
document.querySelector('#dupidlist').textContent = JSON.stringify(dupes);