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
ip: str,
    site_url: str,
    team_id: int,
    event: str,
    distinct_id: str,
    properties: Dict,
    timestamp: Union[datetime.datetime, str],
) -> None:
    elements = properties.get("$elements")
    elements_list = None
    if elements:
        del properties["$elements"]
        elements_list = [
            Element(
                text=el["$el_text"][0:400] if el.get("$el_text") else None,
                tag_name=el["tag_name"],
                href=el["attr__href"][0:2048] if el.get("attr__href") else None,
                attr_class=el["attr__class"].split(" ") if el.get("attr__class") else None,
                attr_id=el.get("attr__id"),
                nth_child=el.get("nth_child"),
                nth_of_type=el.get("nth_of_type"),
                attributes={key: value for key, value in el.items() if key.startswith("attr__")},
            )
            for index, el in enumerate(elements)
        ]
    team = Team.objects.only(