The following JavaScript function will display all the double ID's in the DOM tree of the HTML page using an alert message:

  function findDoubleId() {
    var _tags = document.body.getElementsByTagName('*');
    var _ids = {};
    var _doubledIds = [];
    for (var _index = 0; _index < _tags.length; _index++) {
      var _tag = _tags [_index];
      if (_tag.id) {
        if (! _ids[_tag.id]) {
          ids[_tag.id] = true;
        } else {
          _doubledIds.push(_tag.id);
        }
      }   
    }
    alert('doubled ids: ' + _doubledIds);
  }