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Let's say I have 2 sites /MergePDF?csharp and /MergePDF?java. They have the exact HTML, CSS and JavaScript. The only difference is the difference in content provided by the JavaScript. Is is it possible that a search engine could see that the HTML is the same and not actually check the content of the site before judging whether it's a duplicate?

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    Its likely a moot point. Yhe famed "duplicate content penalty" doesnt really exist, save that Google will prefer 1 version over the other.
    – davidgo
    Jan 7, 2022 at 10:26

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Googlebot now executes JavaScript and indexes the content that is in the document object model (DOM) after JavaScript runs. For Google specifically, the content won't be 100% duplicate anymore. For other search engines like Bing, Yandex and Baidu, your page will appear duplicate because they see it as it was before the JavaScript runs.

Having multiple pages on your site with the same content isn't necessarily an SEO problem. When search engines encounter duplicate pages with your site, they typically just pick one to index and ignore the others. Your entire site won't get a penalty unless that duplicate content is spammy or stolen from external sources. See What is duplicate content and how can I avoid being penalized for it on my site?

I'm a little worried that your JavaScript may just be finding keywords and replacing them. If that is the case, you are implementing what Google calls doorway pages. Lots of similar pages with nearly the same content except with swapped out keywords. If your JavaScript is doing that, then Google could penalize your entire site.

However, if you JavaScript is swapping out code samples and API documentation for various programming languages, you should be fine. Substantially changing the content for different programming languages would be just fine.

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  • So you're saying Google will rerun the JavaScript even if the HTML is the exact same, just the URL is different? Jan 7, 2022 at 19:47
  • BTW, I can confirm my pages are not "doorway pages". I have seen how annoying they are so I understand why Google wouldn't like them. Jan 7, 2022 at 19:59
  • Yes, Googlebot runs the JavaScript. It often takes Google extra weeks or even months to index content that requires JS, but it will do so eventually. Jan 7, 2022 at 20:20
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Hmm good question. I am not and SEO perse but I would just block the search engines on one of the sites with

<meta name="robots" content="noindex, nofollow">

That's probably your best option so it will only crawl the one website.

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  • I want it to crawl both websites though. That way if someone searches "merge PDFs c#", they will get the c# version where if someone searches "merge PDFs java" they get the Java version. The content will be displayed by the JavaScript. Jan 7, 2022 at 19:46

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