Wie Mehrfachbilder in Media Gallery vermeiden?

// get one random image with "Startseite" in title
$args = array(
    'posts_per_page' => 1,
    'post_type' => 'attachment',
    'post_mime_type' => 'image',
    'orderby' => 'rand',
    's' => 'Startseite'
);
$attachments = get_posts( $args );
if ( !empty( $attachments ) ) {
    foreach ( $attachments as $attachment ) {    // actually only one requested
        echo wp_get_attachment_image( $attachment->ID, 'full' );
    }
} else {
    echo '<img class="startseite-dummy" src="' . get_stylesheet_directory_uri() . '/images/startseite-dummy.png" alt="" />';
}