Image not desired showing

I found my problem .. in the function.php I have this code :

function display_featured_image($content) {
	global $post;

	$img_path = get_template_directory_uri().'/inc/images/placeholder.jpg'; // fallback image

	if (is_single()) {
		if (has_post_thumbnail()) {
			the_post_thumbnail();
		} else {
			$attachments = get_posts(array(
				'post_type' => 'attachment',
				'post_mime_type'=>'image',
				'posts_per_page' => 0,
				'post_parent' => $post->ID,
				'order'=>'ASC'
			));
			if ($attachments) {
				foreach ($attachments as $attachment) {
					set_post_thumbnail($post->ID, $attachment->ID);
					break;
				}
				the_post_thumbnail();
			} else {
				$content = '<img src="' . $img_path . '" alt="">' . $content;
			}
		}
	}
	return $content;
}
add_filter('the_content', 'display_featured_image');

for the featured image. I removed it and ti’s work.