WooCommerce: Check if Coupon Already Applied

gistfile1.php

<?php
 
/**
* WooCommerce: Hide 'Coupon form' on checkout page if a coupon was already applied in the cart
* Add to theme functions.php file or Code Snippets plugin
*/

add_filter( 'woocommerce_coupons_enabled', 'woocommerce_coupons_enabled_checkout' );

function woocommerce_coupons_enabled_checkout( $coupons_enabled ) {
    global $woocommerce;
    if ( ! empty( $woocommerce->cart->applied_coupons ) ) {
        return false;
    }
    return $coupons_enabled;
}

Stack Overflow: WooCommerce check if coupon already applied?
http://stackoverflow.com/questions/17626814/woocommerce-check-if-coupon-already-applied