Duplicated JavaScript

Why It Matters

• Double the download, double the waste. Loading the same script file twice means double the bandwidth consumption. On metered or slow connections, this directly hurts page load times and costs real money for users.
• Double the parsing and execution time. The browser does not know the second script is a duplicate — it parses and executes it all over again. This adds unnecessary CPU work that delays interactivity.
• It can cause bugs. Running initialization code twice can produce unexpected side effects — event listeners registered twice, global variables overwritten, or setup routines that conflict with themselves.
• It inflates page weight. Performance audits flag duplicated scripts as wasted bytes. Removing them is one of the easiest ways to reduce total page size and improve load performance.

How to Fix It

1. Audit your script tags. View the page source and check for duplicate <script src="/..."> tags pointing to the same file. This is the most obvious form of duplication and the easiest to fix.
2. Check bundled dependencies. If you use a build tool or bundler, inspect the output for duplicate modules. Two different components importing the same library can result in it being bundled twice if not properly deduplicated.
3. Centralize library loading. Load shared libraries (like utility functions or frameworks) once in a central location rather than letting each plugin or component load its own copy.
4. Use a module system. Modern JavaScript module systems ensure each module is loaded only once regardless of how many files import it. Migrating to ES modules or a proper module bundler eliminates many duplication problems.
5. Review third-party integrations. Plugins, widgets, and embedded scripts from external providers may include their own copies of common libraries. Check what they load and remove duplicates from your own code if the plugin already covers it.

Common Mistakes

• Including libraries in both the template and individual pages. A common pattern is loading a library globally in the site template and also in specific page templates that "need" it. The result is two copies on every page that has both.
• Different versions of the same library. Loading version 3.5 and version 3.6 of the same library is still duplication — with the added risk of version conflicts. Pick one version and stick with it.
• Relying on caching to fix the problem. Browser caching ensures the file is not downloaded twice from the server, but the browser still parses and executes each <script> tag separately. Caching does not prevent duplicate execution.
• Not auditing after plugin updates. Updating plugins can introduce new dependencies that duplicate existing ones. After every plugin update, check your page source for new duplicate scripts.