Duplicate page content should be avoided as you will get less control over how your search results are displayed and how backlinks are consolidated.

If you have a single page accessible by multiple URLs … Google sees these as duplicate versions of the same page. Google will choose one URL as the canonical version and crawl that, and all other URLs will be considered duplicate URLs and crawled less often.

Google, “Consolidate duplicate URLs”

Set canonical URLs

All pages should specify a valid canonical URL to get more control over how duplicate URLs are treated by search engines. When a set of URLs on your site return duplicate or near duplicate content, search engines will select a single definitive URL for that content called the canonical URL. This URL will be crawled more often, will take priority in search results over URLs with duplicate content and search rank boosting backlinks to the URLs with duplicate content will be viewed as linking to the canonical URL. Note that “self-canonicalizing” a page by setting its canonical URL to itself is both valid and useful as it can help eliminate potential duplicates such as when pages may be linked to with tracking URL parameters. To suggest the canonical URL for a page you can 1) add a <link rel="canonical" href="…"> tag inside the page’s <head> tag (most common) or 2) add a Link: <…>; rel="canonical" header to the page’s response headers. Google suggest giving absolute canonical URLs over relative ones. Search engines are likely to ignore your canonical URL suggestion if you 1) include multiple canonical URL suggestions per page or 2) suggest a URL that is broken, redirects, isn’t indexable or isn’t itself canonical. Keep in mind that for exact duplicates you should consider if it is more approriate to use 301 redirects over canonical URLs to consolidate duplicates instead.

Avoid duplicate page content

Every page should provide unique content that doesn’t appear elsewhere on the site. Search engines are likely to choose not to display pages in search results that are too similar as showing duplicates entries in search results is unhelpful to users. Duplicate pages can also reduce the search rank benefit of backlinks because it’s better to have backlinks to a single URL compared to backlinks spread over a set of duplicate page URLs. Crawling duplicates will also use up the resources search crawlers allocate to crawling your site which means important pages might not be indexed. You can eliminate sets of duplicate pages by consolidating them to a single URL using redirects or canonical tags.

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