add_filter( 'wp_unique_post_slug_is_bad_hierarchical_slug', 'portfolio_is_bad_hierarchical_slug', 10, 4 );
function portfolio_is_bad_hierarchical_slug( $is_bad_hierarchical_slug, $slug, $post_type, $post_parent ) {
if ( !$post_parent && $slug == 'completed' )
    return true;
return $is_bad_hierarchical_slug;
}
add_filter( 'wp_unique_post_slug_is_bad_flat_slug', 'portfolio_is_bad_flat_slug', 10, 3 );
function portfolio_is_bad_flat_slug( $is_bad_flat_slug, $slug, $post_type ) {
if ( $slug == 'completed' )
    return true;
return $is_bad_flat_slug;
}