model formsets and recently stale data

[Tom]

We saw an unexpected result working with model formsets and recently stale data. 

Consider a simple Book model with soft deletion.

```

class Book(models.Model):

    name = models.CharField(max_length=100)

    deleted = models.BooleanField(default=False)

```

We have one book

```

book = Book.objects.create(name="Grapes of Wrath")

```

and a model formset to edit Books.

```

BookFormSet = modelformset_factory(Book, fields=('name',), max_num=None, extra=0)

```

A user initializes a formset to edit all non-deleted books. While that user is filling out the edit form, another user deletes the book.

```

# The user calls a view that initializes the formset. 

formset = BookFormSet(queryset=Book.objects.filter(deleted=False))

data = {

            'form-TOTAL_FORMS': '1',

            'form-INITIAL_FORMS': '1',

            'form-MAX_NUM_FORMS': '',

            'form-0-id': book.pk,

            'form-0-name': "New Title"

    }

# Before submission, another user deletes the book. 

book.deleted = True

book.save()

# Now the original user submits the form

formset = BookFormSet(data=data, queryset=Book.objects.filter(deleted=False))

formset.is_valid()  # True

formset.save()

```

Two Books now exist. Despite having `data['form-0-id'] = book.pk` and extra=0, django uses a ModelForm with instance=None, which ends up creating a new row. This is because BaseModelFormSet's _existing_object returns None if the submitted id is not in the queryset's set of ids. https://github.com/django/django/blob/master/django/forms/models.py#L573. 

Is this expected? If an id is submitted with a form, and that id isn't expected, could it make sense for formset validation to fail instead?