Eventengine's modulestate plugin will not handle alerts on modules that have duplicates

[lunkwill42]

There is an unhandled MultipleObjectsReturned exception in the eventengine modulestate plugin.
Observed under NAV 4.6.0, a customer has a device that reports duplicates of all its modules in ENTITY-MIB::entPhysicalTable.

This causes problems for the modulestate plugin in the event engine, as the attempt to fetch the module's corresponding entity returns multiple matches:

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/nav/eventengine/engine.py", line 67, in _decorated
    return func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/nav/eventengine/plugins/delayedstate.py", line 168, in _make_down_warning
    self._post_down_warning()
  File "/usr/lib/python2.7/dist-packages/nav/eventengine/plugins/modulestate.py", line 79, in _post_down_warning
    if self._is_chassis_down():
  File "/usr/lib/python2.7/dist-packages/nav/eventengine/plugins/modulestate.py", line 92, in _is_chassis_down
    chassis = self.get_target().get_chassis()
  File "/usr/lib/python2.7/dist-packages/nav/models/manage.py", line 668, in get_chassis
    me = self.get_entity()
  File "/usr/lib/python2.7/dist-packages/nav/models/manage.py", line 657, in get_entity
    device=self.device)
  File "/usr/lib/python2.7/dist-packages/django/db/models/manager.py", line 92, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/db/models/query.py", line 361, in get
    num if num <= MAX_GET_RESULTS else 'more than %s' % MAX_GET_RESULTS
MultipleObjectsReturned: get() returned more than one NetboxEntity -- it returned 2!