posthog-js supports person.set_once but our backend doesn't. Make sure we can pass through `$set_once` on an `$identify` call, and only set the properties within if the properties aren't already set on the user.
posthog/tasks/test/test_process_event.py
now().isoformat(),
            )
            self.assertEqual(len(get_events()), 1)
            self.assertEqual(get_events()[0].properties["$set"], {"a_prop": "test"})
            person = Person.objects.get()
            self.assertEqual(person.distinct_ids, ["anonymous_id", "new_distinct_id"])
            self.assertEqual(person.properties, {"a_prop": "test"})
            # chec
            process_event(
                "new_distinct_id",
                "",
                "",
                {
                    "event": "$identify",
                    "properties": {
                        "$anon_distinct_id": "anonymous_id",
                        "token": self.team.api_token,
                        "distinct_id": "new_distinct_id",
                    },
                },
                self.team.pk,
                now().isoformat(),
                now().isoformat(),
            )
        # This 
        # 1. U
        # 2. U
        # 3. I
        # Resu
        def test_distinct_with_anonymous_id_which_was_already_created(self) -> None: