Comment_post_ID line appearing twice

I’ve created two themes and both have the same problem. When trying to validate my HTML, I get the error ‘Duplicate ID comment_post_ID.’

I’ve looked at the source code and tracked it down to this line:

<input type='hidden' name='comment_post_ID' value='14' id='comment_post_ID' />

Which appears twice. This is WordPress-generated code, so I’m struggling to understand why it’s happening. It has to be something I’ve done to my comments.php file, right? So if anyone wants to have a look, it’s here:

<?php if(!empty($_SERVER[‘SCRIPT_FILENAME’]) && ‘comments.php’ == basename($_SERVER[‘SCRIPT_FILENAME’])) :
    die( 'This page cannot be viewed independently.' );
    endif;
    if(!empty($post->post_password)) : if($_COOKIE[‘wp-postpass_’ . COOKIEHASH] != $post->post_password) :
    endif;
    endif; ?>

	<?php
	if ( post_password_required() ) {
	echo 'This post is password protected. Enter the password to view comments.';
	return;
	}
	?>
	<?php if ( have_comments() ) : ?>
		<div class="respond">
			<div id="comments">
				<h3>Comments:</h3>
				<ol><?php wp_list_comments(); ?></ol>
			<div class="pages">
				<?php paginate_comments_links(); ?>
			</div>
		<?php endif; ?>

		<?php comment_id_fields();	?>
		<?php comment_form(); ?>
		</div>
	</div>

Thank you so much for any help 🙂

Edit – Right, so taking out this line makes it work:

<?php comment_id_fields(); ?>

I thought that was necessary… could someone let me know if taking that out is going to cause problems?