This may happen to some wordpress blogs that you see some duplicate title tags under Diagnostics >> HTML Suggestions in google webmaster for your blog. As when you reply to comments it is considered as a different URL but it comes with the same content so they detect it as duplicate content, and they are displayed as duplicate title tags under google webmasters as shown in the image below.
If you see these kinds of duplicate title tags links for your website or blog they may not immediately affect your website, but they are definitely not good for any website or blog in the longer run.
In order to fix this issue, google provides a simple setting under Site Configuration >> Settings >> Parameter Handling which allows to you define such settings that could
Under this settings you can specify the action google should take while indexing or crawling your website to ignore links ending or having this parameter in the URL not to index.
By default google was handling the replytocom parameter with the setting “Let Google decide” and it was indexing these pages, but to reduce the duplicate content you should Click the Edit link beside this – and set the option to Ignore as done in the image above.
Another way to solve this issue is to add the following line in your robots.txt file of your website or blog.
Or else you can also use the replytocom redirector plugin which work with the wordpress blogs, which will redirect all these links to the main post article page so that google will not be indexing such pages due to redirection. Similarly you can delete, ref or ref= links which are not useful to you from getting indexed by following the above steps, you can also add an additional parameter which may have noticed and not displayed as suggestion on the parameter handling page.
[ Source Quick Online Tips ]