I am developing a web application and I would like to check for duplicate id values in a HTML page. I am running the application on my local machine.

There is a way to do that?

P.S.: I am using Firefox and Firebug.

4 Answers 4


The W3C's validator tool will report duplicate IDs. To test your code:

  1. Copy the generated source code to your clipboard.
  2. Visit http://validator.w3.org/#validate_by_input
  3. Paste your markup into the box and hit 'Check'.

You can test it with the following code if you wish:

<!doctype html>
        <div id="test">Test div</div>
        <div id="test">Test div 2</div>

This produces the following error:

enter image description here


Run this code on your browser’s JavaScript console:-

(function findDuplicateIds() {
    var ids = {};
    var all = document.all || document.getElementsByTagName("*");
    for (var i = 0, l = all.length; i < l; i++) {
        var id = all[i].id;
        if (id) {
            if (ids[id]) {
                console.log("Duplicate id: #" + id);
            } else {
                ids[id] = 1;
  • Is there a way to check for duplicates of a particular id that avoids iterating through all elements?
    – dlaliberte
    Mar 27, 2019 at 15:57
  • To answer my own question, this appears to work: document.querySelectorAll("[id='test']")
    – dlaliberte
    Mar 27, 2019 at 16:04
  • all is now deprecated. Care to modify? Tweaked this and VERY helpful. Somehow I have turned off duplicate id checking in my IDE and this saved me big time. Thanks
    – BeNice
    Nov 16, 2019 at 15:19
  1. http://validator.w3.org/check

  2. If you have Web Developer Toolbar installed, you can use it to contact the above service directly from browser: Tools -> Validate Local HTML

  3. Some developer tools (like PhpStorm/WebStorm) automatically perform such validation.


Use the W3C Validator. It will tell you if there are duplicated ids.
If your site is not online, use Opera. They have a nice feature that uploads the page in order to validate it.

  • Right click on the page
  • Validate

enter image description here enter image description here

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