teste.php

<?php
$args = array(
  'post_type' => 'post',
  's' => 'Expresso',
  'post_status' => 'publish'
  );
?>
<?php $the_query = new WP_Query( $args ); ?>
<?php $titles = array();?>
<?php if ($the_query->have_posts()): while ($the_query->have_posts()) : $the_query->the_post();?>
<?php if ( in_array( get_the_title(), $titles ) ) {
  continue;
} ?>
<!--Exibir todos os posts com a palavra expresso sem repetição -->
<?php the_title();?>
<?php $titles[] = get_the_title();?> 
<?php endwhile; else:?>
<?php endif;?>