Add safeguards and tests against duplicate cancellations

src/pretix/base/services/invoices.py

@@ -210,6 +210,8 @@ def build_cancellation(invoice: Invoice):
 
 
 def generate_cancellation(invoice: Invoice, trigger_pdf=True):
+    if invoice.refered.exists():
+        raise ValueError("Invoice should not be canceled twice.")
     cancellation = modelcopy(invoice)
     cancellation.pk = None
     cancellation.invoice_no = None

src/pretix/base/services/orders.py

@@ -156,7 +156,7 @@ def mark_order_expired(order, user=None, auth=None):
 
         order.log_action('pretix.event.order.expired', user=user, auth=auth)
         i = order.invoices.filter(is_cancellation=False).last()
-        if i:
+        if i and not i.refered.exists():
             generate_cancellation(i)
 
     order_expired.send(order.event, order=order)