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if ( has_post_thumbnail() ) {
    $large_image_url = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'large' );
    if ( ! empty( $large_image_url[0] ) ) {
        echo '<a href="' . esc_url( $large_image_url[0] ) . '" title="' . the_title_attribute( array( 'echo' => 0 ) ) . '">';
        echo get_the_post_thumbnail( $post->ID, 'thumbnail' ); 
        echo '</a>';
    }
}