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
with freeze_time("2020-01-01T12:00:05.200Z"):
                process_event(
                    "distinct_id",
                    "",
                    "",
                    {"offset": 150, "event": "$autocapture", "distinct_id": "distinct_id",},
                    self.team.pk,
                    now().isoformat(),
                    now().isoformat(),
                )  # s
            event = get_events()[0]
            self.assertEqual(event.timestamp.isoformat(), "2020-01-01T12:00:05.050000+00:00")
        def test_offset_timestamp_no_sent_at(self) -> None:
            with freeze_time("2020-01-01T12:00:05.200Z"):
                process_event(
                    "distinct_id",
                    "",
                    "",
                    {"offset": 150, "event": "$autocapture", "distinct_id": "distinct_id",},
                    self.team.pk,
                    now().isoformat(),
                    None,
                )  # no s
            event = get_events()[0]
            self.assertEqual(event.timestamp.isoformat(), "2020-01-01T12:00:05.050000+00:00")
        def test_alias_merge_properties(self) -> None:
            Person.objects.create(
                team=self.team,
                distinct_ids=["old_distinct_id"],
                properties={"key_on_both": "old value both", "key_on_old": "old value"},