Prevent Duplicate Content On Comment Pages:

The problem.
Duplicate content is an SEO problem for many websites. It can also be a problem for your WordPress blog if you don’t apply a few tricks.

Introduced in version 2.7, paged comments is a great addition to WordPress because it lets bloggers split comments into multiple pages. The only problem, again, is the risk of duplicate content.

Let’s use the new rel="canonical" attribute to prevent duplicate content in paged comments.

The solution.
Simply paste the following code in your function.php file:

function canonical_for_comments() {
	global $cpage, $post;
	if ( $cpage > 1 ) :
		echo "\n";
	  	echo "<link rel='canonical' href='";
	  	echo get_permalink( $post->ID );
	  	echo "' />\n";
	 endif;
}

add_action( 'wp_head', 'canonical_for_comments' );

Code explanation.
First, we create a function to add the rel="canonical" attribute to comment pages, except page 1. Then we hook this function to WordPress’ wp_head() function. As simple as that!