how to find that my written content is unique - in terms of google and user?
I also want to know whether updating the content or user commenting on that article helps in seo?
Updating the content won't have a direct effect on SEO as changing content is not a ranking factor. But if you improve the content it may become relevant for search phrases and it may encourage other webmasters to link to it. Of course, this is assuming the content is significantly improved in the update.