DanceList.get_visible() needs `.distinct()` in query

In some situations, DanceList.get_visible() can return the same dance list several times. This leads to issues if the queryset is the base of a .get() call (e.g., for the dance list detail pages).

Sentry Issue: STRATHSPEY-V

DanceList.MultipleObjectsReturned: get() returned more than one DanceList -- it returned 2!
(2 additional frame(s) were not displayed)
...
  File "django/views/generic/base.py", line 104, in view
    return self.dispatch(request, *args, **kwargs)
  File "django/views/generic/base.py", line 143, in dispatch
    return handler(request, *args, **kwargs)
  File "django/views/generic/detail.py", line 108, in get
    self.object = self.get_object()
  File "django/views/generic/detail.py", line 53, in get_object
    obj = queryset.get()
  File "django/db/models/query.py", line 640, in get
    raise self.model.MultipleObjectsReturned(