Look for duplicate IDs and report them in the console

duplicateIDs.bookmarklet.js

(function(a){var b=[],c="";a("[id]").each(function(c,d){if(a("[id="+this.id+"]").length>1){b.push(this.id)}});if(b.length){c="Druplicate IDs: "+b.join(", ");window.console&&window.console.log?window.console.log(c):window.alert(c)}})(jQuery)

duplicateIDs.js

(function($) {
  var duplicates = [],
      msg = '';
  $('[id]').each(function(i, el) {
    if ($('[id=' + this.id + ']').length > 1) {duplicates.push(this.id);}
  });
  if (duplicates.length) {
    msg = 'Druplicate IDs: ' + duplicates.join(', ');
    window.console && window.console.log ? window.console.log(msg) : window.alert(msg);
  }
})(jQuery);