firefox/browser/base/content/test/general/browser_duplicateIDs.js

function test() {
  var ids = {};
  Array.prototype.forEach.call(
    document.querySelectorAll("[id]"),
    function (node) {
      var id = node.id;
      ok(!(id in ids), id + " should be unique");
      ids[id] = null;
    }
  );
}