Making Sure Comments are Unique

As a site owner, we all the importance of keeping the content on our sites unique so to not devalue our site's ranking in the search engines. There are a number of tools to check the uniqueness of content including CopyScape, Plagium, and CopyGator. I personally like to use Plagium for content that has not yet been published and CopyScape for content that is available online. But what if your comments aren't unique. Probably not as serious as the content itself but still not something desired. If someone posts the same comments on 10 different sites, do you want the same comment posted on your site? No, because it more than likely is relative nor does it add any value to your article. But how do you know if comments are unique?