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/process_event.py
person: Person
    created = False
    if not Person.objects.distinct_ids_exist(team_id=team_id, distinct_ids=[str(distinct_id)]):
        try:
            person = Person.objects.create(team_id=team_id, distinct_ids=[str(distinct_id)])
            created = True
        except IntegrityError:
            person = Person.objects.get(
                team_id=team_id, persondistinctid__team_id=team_id, persondistinctid__distinct_id=str(distinct_id)
            )
            created = False
    else:
        person = Person.objects.get(
            team_id=team_id, persondistinctid__team_id=team_id, persondistinctid__distinct_id=str(distinct_id)
        )
        created = False
    return person, created
def _update_person_properties(team_id: int, distinct_id: str, properties: Dict) -> None: