A bookmarklet to detect multiple ID's on a page

duplicate_id_alerter.js

javascript:(function(){
  var nodes = document.querySelectorAll('*'),
    ids = [],
    duplicates = [];
  for (var i = 0; i < nodes.length; i++) {
    var node = nodes[i];
    if (node.id !== '') {
      if (ids.indexOf(node.id) === -1) {
        ids.push(node.id);
      } else {
        duplicates.push(node.id);
      }
    }
  }
  if (duplicates.length > 0) {
    alert('Duplicate ID\'s found: \n\n#' + duplicates.join('\n#'));
  }
}());