Try this code. This will hide 'Coupon form' on checkout page, if any coupon is already applied from cart
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;
}
Hope this will be helpful