def METHOD_NAME(self):
    tag0 = self._force_tag()
    tag = self._force_tag()
    self._login("inventory.change_tag")
    response = self.client.post(reverse("inventory:update_tag", args=(tag.pk,)),
                                {"name": tag0.name})
    self.assertEqual(response.status_code, 200)
    self.assertTemplateUsed(response, "inventory/tag_form.html")
    self.assertFormError(response.context["form"], "name", "A tag with this name already exists.")