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Duplicate content is one of the most common SEO problems and can have many causes. If, for example, a website is available in versions with and without "www" (domain.com and www.domain.com), then these are each considered as independent pages and in the worst case can both end up in the index of Google and Co., where they then compete with each other.
Session IDs and other parameters in the URLs can also lead to duplicate content being created. Another example are print-optimized pages: These contain the same content as the original, but with a different layout. For Google, however, what counts most are the contents, which are then available twice.
To check whether there is duplicate content of your own page on Google, you can simply perform a "site:" query. With the search for "site:domain.com" all pages of the domain "domain.com" can be displayed in the index. Duplicate contents can thus be recognized quite quickly.
You can do a lot against duplicate content. First of all there would be the setting of a Canonical URL, so to speak the main URL per page. Using the Canonical statement, you can tell search engines which URL to index and that other URLs should be ignored.
SEO plugins like Yoast also help to avoid duplicate content by building URLs in a way that avoids duplication.
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