Find duplicate IDs and print the elements to console

gistfile1.js

var allElements = document.getElementsByTagName("*");
var allIds = {};
var found = false;
for (var i = 0, n = allElements.length; i < n; ++i) {
  var id = allElements[i].id;
  if (id) {
    if (allIds[id] === undefined) {
      allIds[id] = [];
    }
    allIds[id].push(allElements[i]);
  }
}
for (var elemId in allIds) {
  if (allIds.hasOwnProperty(elemId) && allIds[elemId].length > 1) {
    found = true;
    console.warn('Duplicate ID #' + elemId);
    console.warn(allIds[elemId]);
  }
}
if (!found) {
  console.log('No duplicate IDs found');
}