Stripe and PayPal: Issue warning on payments for paid orders

src/pretix/plugins/paypal/signals.py

@@ -48,6 +48,8 @@ def pretixcontrol_action_display(sender, action, request, **kwargs):
         template = get_template('pretixplugins/paypal/action_refund.html')
     elif action.action_type == 'pretix.plugins.paypal.overpaid':
         template = get_template('pretixplugins/paypal/action_overpaid.html')
+    elif action.action_type == 'pretix.plugins.paypal.double':
+        template = get_template('pretixplugins/paypal/action_double.html')
 
     ctx = {'data': data, 'event': sender, 'action': action}
     return template.render(ctx, request)

src/pretix/plugins/paypal/templates/pretixplugins/paypal/action_double.html

@@ -0,0 +1,9 @@
+{% load i18n %}
+
+<p>
+    {% url "control:event.order" organizer=event.organizer.slug event=event.slug code=data.order as ourl %}
+    {% blocktrans trimmed with payment=data.payment order="<a href='"|add:ourl|add:"'>"|add:data.order|add:"</a>"|safe %}
+        The PayPal transaction {{ payment }} has succeeded, but the order {{ order }} has already been paid by other
+        means. Please double check and refund the money via PayPal's interface.
+    {% endblocktrans %}
+</p>

src/pretix/plugins/paypal/views.py

@@ -161,6 +161,14 @@ def webhook(request, *args, **kwargs):
                 'sale': sale['id']
             })
         )
+    elif order.status not in (Order.STATUS_PENDING, Order.STATUS_EXPIRED) and sale['state'] == 'completed' and \
+            order.payment_provider != "paypal":
+        RequiredAction.objects.create(
+            event=event, action_type='pretix.plugins.paypal.double', data=json.dumps({
+                'order': order.code,
+                'payment': sale['parent_payment']
+            })
+        )
     elif order.status in (Order.STATUS_PENDING, Order.STATUS_EXPIRED) and sale['state'] == 'completed':
         try:
             mark_order_paid(order, user=None)