Duplicate content is content that appears on more than one page on a website or on several websites. The problem with duplicate content from an SEO perspective is that search engines do not want to display two listings with the same content in their search results. Duplicate content on the same website can occur because of a mistake or due to the use of parameters in the URL. These parameters alter the URL, but since the landing page is the same, so is the content, and that is considered two different URLs with duplicate content. The solution to duplicate content on a website is either to create a 301 redirect from the duplicate if it is a mistake, to add a canonical tag to point to the main version or to use the parameter handling tools in Google's Search Console and in Bing's Webmaster Tools. If the content is syndicated to other websites, both the source and the web pages with the syndicated content should add a canonical tag pointing to the source page.
More about duplicate content from Google