Comment 8 for bug 772609

>>> getUtility(IBugSet).get(97266).default_bugtask.bug_subscribers
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/srv/launchpad.net/codelines/trunk/lib/lp/bugs/model/bugtask.py", line 320, in bug_subscribers
    self.bug.getIndirectSubscribers()))
  File "/srv/launchpad.net/codelines/trunk/lib/lp/bugs/model/bug.py", line 939, in getIndirectSubscribers
    self.getSubscribersFromDuplicates(recipients, level))
  File "/srv/launchpad.net/codelines/trunk/lib/lp/bugs/model/bug.py", line 992, in getSubscribersFromDuplicates
    info.structural_subscriptions_from_duplicates.subscribers))
  File "/srv/launchpad.net/codelines/trunk/lib/lp/services/propertycache.py", line 116, in __get__
    value = self.populate(instance)
  File "/srv/launchpad.net/codelines/trunk/lib/lp/bugs/model/bug.py", line 2247, in wrapper
    return factory(func(*args, **kwargs))
  File "/srv/launchpad.net/codelines/trunk/lib/lp/bugs/model/bug.py", line 2344, in structural_subscriptions_from_duplicates
    all_duplicate_structural_subscriptions += duplicate_struct_subs
ForbiddenAttribute: ('__iadd__', [<lp.bugs.model.structuralsubscription.StructuralSubscription object at 0xdf0cdac>, <snip a few more>, <lp.bugs.model.structuralsubscription.StructuralSubscription object at 0xe45c1cc>])