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.
ee/clickhouse/process_event.py
distinct_id: str,
    ip: str,
    site_url: str,
    data: dict,
    team_id: int,
    now: datetime.datetime,
    sent_at: Optional[datetime.datetime],
    event_uuid: UUIDT,
) -> None:
    if settings.DEBUG:
        print(f'Logging event {data["event"]} to WAL')
    KafkaProducer().produce(
        topic=KAFKA_EVENTS_WAL,
        data={
            "uuid": str(event_uuid),
            "distinct_id": distinct_id,
            "ip": ip,
            "site_url": site_url,
            "data": json.dumps(data),
            "team_id": team_id,
            "now": now.isoformat(),
            "sent_at": sent_at.isoformat() if sent_at else "",
        },
    )