How can we find duplicate content on website?

Duplicate content is content that appears on the Internet in more than one place.  If you publish your own content in more than one place, you have duplicate content. If you copy someone else’s content onto your site or if they publish yours on their site, that’s duplicate content. If your site contains duplicate content, it is a huge mistake that can hurt your site ranking and can cause you to be penalized by Google. You can find the duplicate content by using plagiarism checkers to make sure your content is viewed as unique. Here I am going to include some tools which is free to use. 
Siteliner: It is a great tool that can check your entire site once a month for duplicate content.
Copyscape: This tool can quickly check the content that you have written which matched with another published content in a matter of seconds. The comparison tool will highlight content that shows up as duplicate, and it will let you know what percentage of your content matches already published content.
Plagspotter: It can identify duplicate pages of content across the web. It’s a great tool for finding plagiarists who have stolen your content. On weekly basis it can monitor your site to identify duplicate content.