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
process_event(
                "new_distinct_id",
                "",
                "",
                {
                    "event": "$create_alias",
                    "properties": {
                        "distinct_id": "new_distinct_id",
                        "token": self.team.api_token,
                        "alias": "old_distinct_id",
                    },
                },
                self.team.pk,
                now().isoformat(),
                now().isoformat(),
            )
            person1 = Person.objects.get(team=self.team, persondistinctid__distinct_id="old_distinct_id")
            person2 = Person.objects.get(team=self.team, persondistinctid__distinct_id="new_distinct_id")
            self.assertEqual(person1, person2)
            self.assertEqual(len(get_events()), 1)
            self.assertEqual(Person.objects.get().distinct_ids, ["new_distinct_id", "old_distinct_id"])
        def test_alias_both_existing(self) -> None: