def METHOD_NAME(self, exclude=None):
    existing = Page.objects.filter(slug=self.slug, parent=self.parent)
    # We could be updating the page, so don't fail if the existing
    # entry is this page.
    if existing.count() > 1 or (existing.count() == 1 and
                                existing.first().pk != self.pk):
        raise ValidationError(
            {
                NON_FIELD_ERRORS: [
                    _("Duplicate parent/slug combination."),
                ],
            })
    return super().METHOD_NAME(exclude)