Duplicate content occurs when one or more URLs share the same or very similar content.
Most search engines don't have a "duplicate content penalty" in their algorithms, duplicate content can still cause a major headache for your SEO success.
Dangers of Duplicate Content
Search engines may not know which version of the page to rank for a keyword – If there are multiple URLs with similar or identical content, search engines may get confused which could cause your URLs to get filtered or rank poorly.
Wasted Crawl Budget – Search engines do not have unlimited resources, so they usually set limits on how much of your site they're willing to crawl. The number of pages they crawl on your site is referred to as the "crawl budget", and like any limited resource, it must be rationed and prioritized. Ideally, we want search engines to crawl and index the most important pages on your site before crawling or indexing less-important or duplicate pages. Therefore, if there is much duplicate content on your site, search engines could use the entire crawl budget on a small number of unique pages and the rest on a large number of duplicate versions of each of them. Sites with only a few hundred or thousand URLs don't need to worry about this, but for larger sites, this can be a big issue.