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<?php
if ( $post->post_parent ) {
    $children = wp_list_pages( array(
        'title_li' => '',
        'child_of' => $post->post_parent,
        'echo'     => 0
    ) );
} else {
    $children = wp_list_pages( array(
        'title_li' => '',
        'child_of' => $post->ID,
        'echo'     => 0
    ) );
}
if ( $children ) : ?>
    <ul>
        <?php echo $children; ?>
    </ul>
<?php endif; ?>