Fix duplicate charge issue

The problem was that an extra gateway class was instantiated that caused
payments to be processed twice. By removing the extra instance of the
payment gateway the problem has been fixed.

Author: Kristófer R

includes/admin/class-wc-stripe-inbox-notes.php

@@ -55,7 +55,7 @@ public static function create_upe_notes() {
 		WC_Stripe_UPE_Availability_Note::init();
 
 		require_once WC_STRIPE_PLUGIN_PATH . '/includes/notes/class-wc-stripe-upe-stripelink-note.php';
-		WC_Stripe_UPE_StripeLink_Note::init( new WC_Stripe_UPE_Payment_Gateway() );
+		WC_Stripe_UPE_StripeLink_Note::init( WC_Stripe::get_instance()->get_main_stripe_gateway() );
 	}
 
 	public static function get_campaign_2020_cutoff() {

includes/notes/class-wc-stripe-upe-stripelink-note.php

@@ -67,12 +67,12 @@ private static function get_note_class() {
 	/**
 	 * Init Link payment method notification
 	 *
-	 * @param WC_Stripe_UPE_Payment_Gateway $gateway
+	 * @param WC_Stripe_Payment_Gateway $gateway
 	 *
 	 * @return void
 	 * @throws \Automattic\WooCommerce\Admin\Notes\NotesUnavailableException
 	 */
-	public static function init( WC_Stripe_UPE_Payment_Gateway $gateway ) {
+	public static function init( WC_Stripe_Payment_Gateway $gateway ) {
 		if ( ! WC_Stripe_Feature_Flags::is_upe_checkout_enabled() ) {
 			return;
 		}
@@ -84,6 +84,10 @@ public static function init( WC_Stripe_UPE_Payment_Gateway $gateway ) {
 			return;
 		}
 
+		if ( ! is_a( $gateway, 'WC_Stripe_UPE_Payment_Gateway' ) ) {
+			return;
+		}
+
 		// If store currency is not USD, skip
 		if ( 'USD' !== get_woocommerce_currency() ) {
 			return;

woocommerce-gateway-stripe.php

@@ -63,14 +63,14 @@ class WC_Stripe {
 			/**
 			 * The *Singleton* instance of this class
 			 *
-			 * @var Singleton
+			 * @var WC_Stripe
 			 */
 			private static $instance;
 
 			/**
 			 * Returns the *Singleton* instance of this class.
 			 *
-			 * @return Singleton The *Singleton* instance.
+			 * @return WC_Stripe The *Singleton* instance.
 			 */
 			public static function get_instance() {
 				if ( null === self::$instance ) {