One of my site ranking down in Google. I checked for the reason then I found my site have 2 versions i.e www and without www. Also when I check my site content for duplicate issue then also it shows another site having the same content. But since the last 2 years site well ranked in google. My site content is unique, but how can I prove or shall I change the content. Due to which google's algorithm (panda and Penguin) had affect my site?
Your first point indicate the canonical error, you can easily remove this error from the webmaster or form the code. And in your second point you mentioned about the content, in the google panda update duplicate content is restricted, it doesn't matter that you use the duplicate content or some body use your content in their site. In my point of view you need to change your website content to get the previous rank. And try to resolve the Canonical error. Thanks,
Your website must redirect from without WWW URL to WWW URL as this is the canonical error, so you have to remove it from your website as if you don't remove this error,, google crawl it as 2 separate websites and this can be the reason for Copied Content shown in your website. So better to focus on Canonical error, Copied Content problem will be resolved by its own.. Hope this help ..!!