duplicate_ids.js

  // Find duplicate ids
  var allElements = document.getElementsByTagName("*"), allIds = {}, dupIDs = [];
  for (var i = 0, n = allElements.length; i < n; ++i) {
    var el = allElements[i];
      if (el.id) {
        if (allIds[el.id] !== undefined) {
          dupIDs.push(el.id);
        }
        allIds[el.id] = el.name || el.id;
      }
  }

  if (dupIDs.length) {
      console.error("Duplicate ID's:", dupIDs);
  } else {
      console.log("No Duplicates Detected");
  }

From http://ryanguill.com/js/html/dom/2016/02/16/dom-find-duplicate-ids.html