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
)
            Person.objects.create(
                team=self.team,
                distinct_ids=["new_distinct_id"],
                properties={"key_on_both": "new value both", "key_on_new": "new value"},
            )
            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(),
            )
            self.assertEqual(len(get_events()), 1)
            person = Person.objects.get()
            self.assertEqual(person.distinct_ids, ["old_distinct_id", "new_distinct_id"])
            self.assertEqual(
                person.properties,
                {"key_on_both": "new value both", "key_on_new": "new value", "key_on_old": "old value",},
            )
        def test_long_htext(self) -> None: