If tag exists, then echo once

Question

I have the following code and I'm trying to check to see if a specific tag exists, and if so, I need to echo it once. The problem is when the tag exists in multiple places, it echos multiple times.

// LOOP ARGUMENTS
$args = array(
    'post_type' => 'xyz_members', 
    'posts_per_page' => -1, 
    'orderby' => 'title', 
    'order' => 'ASC', 
    'tax_query' => array(
        array(
            'taxonomy' => 'member_types',
            'field' => 'slug',
            'terms' => 'current-class',
            'operator' => 'NOT IN'
        )
    )
);
$loop = new WP_Query( $args );
while ( $loop->have_posts() ) : $loop->the_post();                              
    // CUSTOM CONTENT
    $postTags = get_the_tags();
    if ($postTags) while($postTags as $tag) {
        if ($tag->slug == '2006') { ?><li id="_2013" class="filter on">2006</li><?php }
        if ($tag->slug == '2007') { ?><li id="_2013" class="filter on">2007</li><?php }
        if ($tag->slug == '2008') { ?><li id="_2013" class="filter on">2008</li><?php }
        if ($tag->slug == '2009') { ?><li id="_2013" class="filter on">2009</li><?php }
        if ($tag->slug == '2010') { ?><li id="_2013" class="filter on">2010</li><?php }
        if ($tag->slug == '2011') { ?><li id="_2013" class="filter on">2011</li><?php }
        if ($tag->slug == '2012') { ?><li id="_2013" class="filter on">2012</li><?php }
        if ($tag->slug == '2013') { ?><li id="_2013" class="filter on">2013</li><?php }
        if ($tag->slug == '2014') { ?><li id="_2013" class="filter on">2014</li><?php }
        if ($tag->slug == '2015') { ?><li id="_2013" class="filter on">2015</li><?php }
        if ($tag->slug == '2016') { ?><li id="_2013" class="filter on">2016</li><?php }
        if ($tag->slug == '2017') { ?><li id="_2013" class="filter on">2017</li><?php }
        if ($tag->slug == '2018') { ?><li id="_2013" class="filter on">2018</li><?php }
        if ($tag->slug == '2019') { ?><li id="_2013" class="filter on">2019</li><?php }
        if ($tag->slug == '2020') { ?><li id="_2013" class="filter on">2020</li><?php }
    }
/* END WHILE AND RESET QUERY */ 
endwhile; wp_reset_query();

Answer 1

I'm not sure what you want, but if you are trying to get a unique <li>-list from the query you can try (untested):

<?php

// LOOP ARGUMENTS
$args = array( 'post_type' => 'xyz_members', 'posts_per_page' => -1, 'orderby' => 'title', 'order' => 'ASC', 'tax_query' => array(array('taxonomy' => 'member_types','field' => 'slug','terms' => 'current-class','operator' => 'NOT IN')));
$loop = new WP_Query( $args );
$tags = array();

// COLLECT 
while ( $loop->have_posts() ) : $loop->the_post();
    $postTags = get_the_tags();
    if ($postTags):
        while($postTags as $tag):
            $tags[$tag->slug] = $tag->slug; 
        endwhile;
    endif;
endwhile; 

// OUTPUT
$years = range( 2006, 2020 );
foreach( $years as $year ):
    $year = (string) $year;
    if( isset( $tags[$year] ) )
         printf('<li id="_%s" class="filter on">%s</li>', $tags[$year], $tags[$year] );

endforeach;

?>

You could also consider array_unique() to remove duplicate values from an array.