Two "leave reply" boxes at the bottom. How to remove one?

Looks like you have duplicate markup for comments. I would check out your single.php file or comments.php file to see if something is duplicated in one of these template files causing the issue for displaying comments twice. Hope that helps.

That’s the single.php file.

<?php
/**
 * The Template for displaying all single posts.
 *
 * @package ThemeGrill
 * @subpackage Radiate
 * @since Radiate 1.0
 */

get_header(); ?>

	<div id="primary" class="content-area">
		<main id="main" class="site-main" role="main">

		<?php while ( have_posts() ) : the_post(); ?>

			<?php get_template_part( 'content', 'single' ); ?>

			<?php radiate_post_nav(); ?>

			<?php
				// If comments are open or we have at least one comment, load up the comment template
				if ( comments_open() || '0' != get_comments_number() ) :
					comments_template();
				endif;
			?>

		<?php endwhile; // end of the loop. ?>

		</main><!-- #main -->
	</div><!-- #primary -->

<?php get_sidebar(); ?>
<?php get_footer(); ?>

And here’s comments.php

<div id="comments" class="comments-area">

	<?php // You can start editing here -- including this comment! ?>

	<?php if ( have_comments() ) : ?>
		<h2 class="comments-title">
			<?php
				printf( _n( 'One thought on &ldquo;%2$s&rdquo;', '%1$s thoughts on &ldquo;%2$s&rdquo;', get_comments_number(), 'radiate' ),
					number_format_i18n( get_comments_number() ), '<span>' . get_the_title() . '</span>' );
			?>
		</h2>

		<?php if ( get_comment_pages_count() > 1 && get_option( 'page_comments' ) ) : // are there comments to navigate through ?>
		<nav id="comment-nav-above" class="comment-navigation" role="navigation">
			<h1 class="screen-reader-text"><?php _e( 'Comment navigation', 'radiate' ); ?></h1>
			<div class="nav-previous"><?php previous_comments_link( __( '&larr; Older Comments', 'radiate' ) ); ?></div>
			<div class="nav-next"><?php next_comments_link( __( 'Newer Comments &rarr;', 'radiate' ) ); ?></div>
		</nav><!-- #comment-nav-above -->
		<?php endif; // check for comment navigation ?>

		<ul class="comment-list">
			<?php
				wp_list_comments( array(
					'callback'    => 'radiate_comment',
					'short_ping'  => true
				) );
			?>

		</ul><!-- .comment-list -->

		<?php if ( get_comment_pages_count() > 1 && get_option( 'page_comments' ) ) : // are there comments to navigate through ?>
		<nav id="comment-nav-below" class="comment-navigation" role="navigation">
			<h1 class="screen-reader-text"><?php _e( 'Comment navigation', 'radiate' ); ?></h1>
			<div class="nav-previous"><?php previous_comments_link( __( '&larr; Older Comments', 'radiate' ) ); ?></div>
			<div class="nav-next"><?php next_comments_link( __( 'Newer Comments &rarr;', 'radiate' ) ); ?></div>
		</nav><!-- #comment-nav-below -->
		<?php endif; // check for comment navigation ?>

	<?php endif; // have_comments() ?>

	<?php
		// If comments are closed and there are comments, let's leave a little note, shall we?
		if ( ! comments_open() && '0' != get_comments_number() && post_type_supports( get_post_type(), 'comments' ) ) :
	?>
		<p class="no-comments"><?php _e( 'Comments are closed.', 'radiate' ); ?></p>
	<?php endif; ?>

	<?php comment_form(); ?>

</div><!-- #comments -->

That all looks ok. What theme are you using? Where did you download it from?

I use the Radiate theme.
I had just an update of the theme and…the second field disappeared ??