Prevents customers from purchasing multiple subscriptions
<?php
/*
 * Plugin Name: Easy Digital Downloads - Single Subscription
 * Description: Prevents customers from purchasing multiple subscriptions
 * Author: Easy Digital Downloads
 * Author URI: https://easydigitaldownloads.com/
 * Version: 1.0
 */

if ( ! defined( 'ABSPATH' ) )
	exit;

function pw_edd_recurring_limit_one_subscription( $valid_data, $post_data ) {
	
	if( ! class_exists( 'EDD_Recurring_Customer' ) ) {
		return;
	}

	if( ! is_user_logged_in() ) {
		return;
	}

	$purchase_data = array(
		'downloads' => edd_get_cart_contents()
	);

	$subscriber = new EDD_Recurring_Subscriber( get_current_user_id(), true );
	$existing = $subscriber->has_active_subscription();

	if( $existing && EDD_Recurring()->is_purchase_recurring( $purchase_data ) ) {
		edd_set_error( 'edd-one-subscription', __( 'You already have an active subscription so may not purchase a second one.', 'edd' ) );
	}

}
add_action( 'edd_checkout_error_checks', 'pw_edd_recurring_limit_one_subscription', 10, 2 );
To implement this snippet on your site, create a brand new, empty WordPress plugin and paste the above code inside.