Twenty seventeen adds a featured image to the page by default, so you are seeing that in addition to the zoomable one added by the plugin.
This is added by the following lines in the theme’s header.php:
/*
* If a regular post or page, and not the front page, show the featured image.
* Using get_queried_object_id() here since the $post global may not be set before a call to the_post().
*/
if ( ( is_single() || ( is_page() && ! twentyseventeen_is_frontpage() ) ) && has_post_thumbnail( get_queried_object_id() ) ) :
echo '<div class="single-featured-image-header">';
echo get_the_post_thumbnail( get_queried_object_id(), 'twentyseventeen-featured-image' );
echo '</div><!-- .single-featured-image-header -->';
endif;
?>
To override this I would suggest using a child theme with a custom version of this file omitting these lines