OptinMonster single-optin for MailChimp as a plugin.

optin-monster-mailchimp-single-optin.php

<?php
/**
 * Plugin Name: OptinMonster MailChimp Single Optin
 * Plugin URI: http://optinmonster.com/docs/how-to-enable-single-optin-for-mailchimp-in-optinmonster/
 * Description: Enable single optin for MailChimp.
 * Version: 1.0.0
 * Author: Erica Franz
 * Author URI: https://fatpony.me
 * License: MIT
 */

defined( 'ABSPATH' ) or die( 'No script kiddies please!' );

add_filter( 'optin_monster_mailchimp_double', '__return_false' );