Not sure how long WordPress has been doing this, but there is a check for duplicate comments. If the same user/email posts the same exact comment on the same post, the user will get a message like:

Duplicate comment detected; it looks as though you’ve already said that!

This is great actually, and keeps people from submitting the same comment twice if they get impatient waiting for moderation or otherwise click that submit button twice.

As a blog owner and WordPress developer, there may be situations where you want to allow people to post the same comment. If you want to enable duplicate comments on your blog for some reason, you can use this code here. Just add it to your theme’s functions.php or put this in a .php in your plugins folder and enable it.

function enable_duplicate_comments_preprocess_comment($comment_data)
	//add some random content to comment to keep dupe checker from finding it
	$random = md5(time());	
	$comment_data['comment_content'] .= "disabledupes{" . $random . "}disabledupes";	
	return $comment_data;
add_filter('preprocess_comment', 'enable_duplicate_comments_preprocess_comment');
function enable_duplicate_comments_comment_post($comment_id)
	global $wpdb;
	//remove the random content
	$comment_content = $wpdb->get_var("SELECT comment_content FROM $wpdb->comments WHERE comment_ID = '$comment_id' LIMIT 1");	
	$comment_content = preg_replace("/disabledupes\{.*\}disabledupes/", "", $comment_content);
	$wpdb->query("UPDATE $wpdb->comments SET comment_content = '" . $wpdb->escape($comment_content) . "' WHERE comment_ID = '$comment_id' LIMIT 1");
		add your own dupe checker here if you want
add_action('comment_post', 'enable_duplicate_comments_comment_post');

For reference, here is the dupe check code in wp-includes/comment.php. A newer version should probably have a hook above and/or below to allow people to override the dupe checker more directly.

// Simple duplicate check
	// expected_slashed ($comment_post_ID, $comment_author, $comment_author_email, $comment_content)
	$dupe = "SELECT comment_ID FROM $wpdb->comments WHERE comment_post_ID = '$comment_post_ID' AND comment_approved != 'trash' AND ( comment_author = '$comment_author' ";
	if ( $comment_author_email )
		$dupe .= "OR comment_author_email = '$comment_author_email' ";
	$dupe .= ") AND comment_content = '$comment_content' LIMIT 1";
	if ( $wpdb->get_var($dupe) ) {
		do_action( 'comment_duplicate_trigger', $commentdata );
		if ( defined('DOING_AJAX') )
			die( __('Duplicate comment detected; it looks as though you’ve already said that!') );
		wp_die( __('Duplicate comment detected; it looks as though you’ve already said that!') );