How do I use `posts_distinct` correctly?

You can see a practical example here: https://wordpress.stackexchange.com/a/142902/19105

Why do you need to use DISTINCT in this situation though? The WP_Query you provided shouldn’t be giving duplicates unless there is a plugin causing trouble.

But I’ll try to answer it anyway. I would suggest keeping the function in functions.php. But move the filter to archive.php around the WP_Query call, like so:

add_filter('posts_distinct', 'search_distinct');
$the_query = new WP_Query( $args );
remove_filter('posts_distinct', 'search_distinct');

If you do this, I also recommend you leave a comment by the search_distinct function. Just explain that the function is used in archive.php — it will help you out in the future if you forget what that function is from.

Also, remove the line: search_distinct(); //Correct?

That is not correct, you typically do not call action/filter functions directly. That is what the “add_filter()” is for.

Complete code:

// functions.php
// Used as a filter for archive.php to eliminate duplicate posts
function search_distinct() { 
    return "DISTINCT"; 
}

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

    // Get posts, make them distinct. search_distinct is defined in functions.php
    add_filter('posts_distinct', 'search_distinct');
    $the_query = new WP_Query( $args );
    remove_filter('posts_distinct', 'search_distinct');

    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(); ?>

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