Use CachedTicket.get_or_create to avoid duplicates

src/pretix/base/migrations/0041_auto_20161018_1654.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.2 on 2016-10-18 16:54
+from __future__ import unicode_literals
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0040_u2fdevice'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='cachedticket',
+            name='cachedfile',
+            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.CASCADE, to='pretixbase.CachedFile'),
+        ),
+    ]

src/pretix/base/models/orders.py

@@ -304,7 +304,7 @@ class Order(LoggedModel):
 
 class CachedTicket(models.Model):
     order = models.ForeignKey(Order, on_delete=models.CASCADE)
-    cachedfile = models.ForeignKey(CachedFile, on_delete=models.CASCADE)
+    cachedfile = models.ForeignKey(CachedFile, on_delete=models.CASCADE, null=True)
     provider = models.CharField(max_length=255)