I've scoured the interwebs but can't find a good working example or discussion of the filter posts_distinct I'm using Advanced Custom Fields but I don't think that could be the problem. Anyway, nothing I try is working.

1. Is where I have my function call correct? Or does it go ahead of the WP_Query?
2. Do I have to pass any arguments?

//functions.php
function search_distinct() { 
    return "DISTINCT"; 
}
add_filter('posts_distinct', 'search_distinct');

//archive.php
<?php
    $args = array(
       'post_type' => 'homily',
       'posts_per_page' => -1,
       'meta_key' => 'homilist',
       'orderby' => 'meta_value', 
       'order' => 'ASC'
    );
    $the_query = new WP_Query( $args );

    search_distinct(); //Correct?

    if ( $the_query->have_posts() ) {
       echo '<ul>';
           while ( $the_query->have_posts() ) {
              $the_query->the_post();
                    echo '<li>' .the_field('homilist') . '</li>';
                }
        echo '</ul>';
     }
     wp_reset_postdata(); ?>

sudo fgs

I've scoured the interwebs but can't find a good working example or discussion of the filter posts_distinct I'm using Advanced Custom Fields but I don't think that could be the problem. Anyway, nothing I try is working.

1. Is where I have my function call correct? Or does it go ahead of the WP_Query?
2. Do I have to pass any arguments?

//functions.php
function search_distinct() { 
    return "DISTINCT"; 
}
add_filter('posts_distinct', 'search_distinct');

//archive.php
<?php
    $args = array(
       'post_type' => 'homily',
       'posts_per_page' => -1,
       'meta_key' => 'homilist',
       'orderby' => 'meta_value', 
       'order' => 'ASC'
    );
    $the_query = new WP_Query( $args );

    search_distinct(); //Correct?

    if ( $the_query->have_posts() ) {
       echo '<ul>';
           while ( $the_query->have_posts() ) {
              $the_query->the_post();
                    echo '<li>' .the_field('homilist') . '</li>';
                }
        echo '</ul>';
     }
     wp_reset_postdata(); ?>

Update: Ended up with a less WordPress-esque solution than I had hoped for, the kind suggestions notwithstanding:

<?php
    global $wpdb;
    $querystr = "SELECT DISTINCT meta_value 
                FROM $wpdb->postmeta 
                WHERE meta_key = 'homilist' 
                ORDER BY meta_value ASC";
    $homilists = $wpdb->get_results( $querystr, OBJECT);
    
    echo '<select id="homilist">';
    for ($i = 1; $i < count($homilists); $i++) {
        echo '<option value="'.$homilists[$i]->meta_value.'">'.$homilists[$i]->meta_value.'</option>';
    }
    echo '</select>'; 
?>