Make sure first period discounts set via the initial payment value can only be used once in Paid Memberships Pro

pmpro_checkout_level_initial_payments_once.php

/**
 * Make sure first period discounts can only be used once.
 * 
 * If a user already checked out for another level in this group,
 * set the initial payment to match the billing amount.
 *
 * Edit the array of level_ids below and then add this code into a custom plugin.
 */
function pmpro_checkout_level_initial_payments_once($level) {
	if( !is_user_logged_in() ) {
		return $level;
	}
	
	global $current_user, $wpdb;
	
	$level_ids = array(1,2,3,4,5,6,7,8);	//need to add the id of each level this should apply to here
	
	$sqlQuery = "SELECT id FROM $wpdb->pmpro_membership_orders WHERE user_id = '" . esc_sql($current_user->ID) . "' AND membership_id IN('" . implode("', '", $level_ids) . "') AND status NOT IN('refunded', 'review', 'token', 'error') LIMIT 1";
	$old_order_id = $wpdb->get_var($sqlQuery);
		
	if( !empty( $old_order_id ) && !empty((float)$level->billing_amount) ) {
		$level->initial_payment = $level->billing_amount;
	}
		
	return $level;
}
add_filter('pmpro_checkout_level', 'pmpro_checkout_level_initial_payments_once', 5);