calendar: Handle duplicates

calendar_utils/models.py

@@ -130,7 +130,11 @@ class CalendarMixin(models.Model):
                     self.calendar.url = self.calendar_url
                     self.calendar.save()
             else:
-                self.calendar, _ = Calendar.objects.get_or_create(url=self.calendar_url)
+                calendar = Calendar.objects.filter(url=self.calendar_url).first()
+                if calendar:
+                    self.calendar = calendar
+                else:
+                    self.calendar = Calendar.objects.create(url=self.calendar_url)
 
             try:
                 self.calendar.update_source()