your main problem is
<?php query_posts('post_type=case_studies'); if ( have_posts() ) while ( have_posts() ) : the_post(); ?>
This means if there are posts, then while there are posts, get from the current post. After processing, it will just keep iterating through the posts.
Delete this line and the end while line.
Instead, use this
<p><?php echo get_post_meta(get_the_ID(), "_Description", true); ?></p>
This gets the current id of the current post.
Having said that, please do some validation of the results you get from get_post_meta to ensure that there are results before echoing it out.