Duplicate ID Checker

Why Duplicate IDs Are a Problem

HTML specification requires that every id attribute value must be unique within the document. Duplicate IDs break CSS selectors, JavaScript targeting, and accessibility features.

Common Issues Caused by Duplicate IDs

• JavaScript Conflicts – document.getElementById() only returns the first element; subsequent calls fail silently
• CSS Selector Problems – ID selectors match the first element only, leaving others unstyled
• Accessibility Breaks – Screen readers and assistive tech rely on unique IDs for navigation
• Invalid HTML – Violates HTML5 specification, may cause validation failures
• Fragment Identifiers – Links with anchors like #section may target wrong elements
• SEO Issues – Invalid markup can affect crawling and indexing

How to Fix Duplicate IDs

Option 1: Make IDs Unique

Add suffixes or modify IDs to be unique:

/* WRONG */





/* RIGHT */

Option 2: Use Classes Instead

If you have repeated elements, use classes instead of IDs:

/* WRONG - duplicate IDs */


Product 1
Product 2

/* RIGHT - use classes for repeated elements */
Product 1
Product 2

Option 3: Use Data Attributes

Use data attributes for JavaScript targeting:

<div class="card" data-card-id="product-1"></div>




/* In JavaScript */
const card = document.querySelector(’[data-card-id=“product-1”]’);

ID Naming Best Practices

• Use meaningful names – id="main-header" instead of id="header1"
• Use kebab-case – id="user-profile" (hyphens), not camelCase for IDs
• Avoid generic names – Don't use id="content", id="section", etc.
• Reserve IDs for unique elements – Use classes for repeated elements
• Keep IDs short – But still descriptive
• Avoid numbers at the end – Unless they're meaningful (not id="div1", id="div2")

Frequently Asked Questions

Is my HTML code private?

Yes. All analysis runs locally in your browser. No HTML code is uploaded or stored anywhere.

How does this tool find duplicates?

The tool uses regex to extract all id attributes from your HTML, then checks which values appear more than once. It tracks exact locations and occurrence counts.

Should I always use unique IDs?

Yes, within a single document. IDs are meant for unique elements. For repeated elements, use classes instead. You can have many elements with the same class, but only one element per ID.

What's the difference between IDs and classes?

ID identifies a single unique element. Class identifies a group of similar elements. Multiple elements can share a class; no two elements can share an ID in valid HTML.

Does this checker work with templates?

Yes. The tool analyzes raw HTML, so it works with static HTML, template markup, and generated code.

What about minified HTML?

The tool works with minified, prettified, or any HTML format.