Webmasters live in fear of the "duplicate content penalty." The myth is that having two URLs on your site that show the same content is an SEO disaster. It will cause the rankings of your entire site to plummet.
In reality, Google almost never penalizes for duplicate content. When Google finds two URLs with the same or very similar content, it usually chooses just one of them to index, but doesn't change the rankings of other pages on the site.
The worst outcome is that Google chooses to index one of the two duplicate URLs, and you would prefer that Google instead index the other URL. That can be fixed with one of several techniques:
- Redirect one of the duplicate URLs to the other
- Use meta canonical tags to tell Google which URL is preferred
- Include only the preferred URL in your XML sitemap
Google will penalize scraper sites that steal content and republish it without permission. Duplicating content from other sites without permission or attribution is likely to get an entire site thrown out of the search results. However internal duplication of your own content doesn't lead search engines to downgrade rankings.
It can also cause SEO problems when content is duplicated on a nearly infinite number of URLs as opposed to just a handful of URLs. A site that uses a session IDs in the URL that changes for every visitor is going to have this problem. Massive numbers of URLs with the same content can make a site uncrawlable. If search engines can't crawl the site, they usually can't index it or assign link juice for rankings.
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This article was written as part of a series about SEO myths.