How to avoid duplicating same meta_value?

Question

I have "publication_year" as meta_key for some images/attachments. I want to list all meta_values inside html table, but I don't want same publication_year value to repeat, my code is:

<?php $args = array( 'post_type' => 'attachment',
                     'post_mime_type' => 'image',
                     'numberposts' => -1,
                     'orderby' => 'menu_order',
                     'order' => ASC
                     );
$attachments = get_posts( $args );
echo '<table id="bibliography">';
if ($attachments) {
foreach ( $attachments as $post ) {
    setup_postdata($post);
    echo '<tr><td>';
    if ( get_post_meta( $post->ID, '_publication_year', true )) {
         echo get_post_meta( $post->ID, '_publication_year', true );
        };
    echo '</td><td>';
    echo '<a href="';
    the_permalink();
    echo '">';
    echo get_the_excerpt();
    echo '</a></td></tr>';
    }
}
echo '</table>';

and with it I get this:

2002 imageDesc1
2003 imageDesc2
2003 imageDesc3
2003 imageDesc4
2004 imageDesc5

but I want this:

2002 imageDesc1
2003 imageDesc2
     imageDesc3
     imageDesc4
2004 imageDesc5

So if there are attachments with same publication years, I want them to group and not repeat How can I avoid duplicating same meta_value?