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
try:
        person = Person.objects.get(
            team_id=team_id, persondistinctid__team_id=team_id, persondistinctid__distinct_id=str(distinct_id)
        )
    except Person.DoesNotExist:
        try:
            person = Person.objects.create(team_id=team_id, distinct_ids=[str(distinct_id)])
        
        except:
            person = Person.objects.get(
                team_id=team_id, persondistinctid__team_id=team_id, persondistinctid__distinct_id=str(distinct_id)
            )
    person.properties.update(properties)
    person.save()
def _set_is_identified(team_id: int, distinct_id: str, is_identified: bool = True) -> None: