Duplicate posts appearing

Hi! I’m customizing the Expound theme, and I put a loop so I could show a single complete post from a certain category. It’s the first post at left of the Widget Area, below Rodin’s Thinker: https://noblogdododo.com

I’ve excluded the category from all that posts above, but I can’t exclude the whole category from the posts below that complete post. But I’d like to prevent the post from appearing again right below the complete post.

I found this solution
https://wpengineer.com/1719/filter-duplicate-posts-in-the-loop/
So I put this code in my added loop
$do_not_duplicate[] = $post->ID;

And now I’m supposed to add this code in the main loop
'post__not_in' => $do_not_duplicate

I found the beginning of the loop in the index.php file, right after I put my “custom” loop, but I don’t know where to put that second line of code. I’m pasting the code of what I suppose it’s the theme’s main loop:

<?php if ( have_posts() ) : ?>

			<?php /* Start the Loop */ ?>
			<?php while ( have_posts() ) : the_post(); ?>

				<?php
					/* Include the Post-Format-specific template for the content.
					 * If you want to overload this in a child theme then include a file
					 * called content-___.php (where ___ is the Post Format name) and that will be used instead.
					 */
					get_template_part( 'content', get_post_format() );
				?>

			<?php endwhile; ?>

			<?php expound_content_nav( 'nav-below' ); ?>

		<?php elseif ( ! is_home() || is_paged() ) : ?>

			<?php get_template_part( 'no-results', 'index' ); ?>

		<?php else : ?>

			<?php
				$featured_posts = expound_get_featured_posts();
				if ( ! $featured_posts->have_posts() )
					get_template_part( 'no-results', 'index' );
			?>

		<?php endif; ?>

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

Thank you!