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
old_person: Optional[Person] = None
    new_person: Optional[Person] = None
    try:
        old_person = Person.objects.get(
            team_id=team_id, persondistinctid__team_id=team_id, persondistinctid__distinct_id=previous_distinct_id
        )
    except Person.DoesNotExist:
        pass
    try:
        new_person = Person.objects.get(
            team_id=team_id, persondistinctid__team_id=team_id, persondistinctid__distinct_id=distinct_id
        )
    except Person.DoesNotExist:
        pass
    if old_person and not new_person:
        try:
            old_person.add_distinct_id(distinct_id)
        
        except IntegrityError:
            if retry_if_failed:  
                _alias(previous_distinct_id, distinct_id, team_id, False)
        return