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/test/test_process_event.py
self._create_user("james")
            Person.objects.create(team=self.team, distinct_ids=["asdfasdfasdf"])
            right_now = now()
            tomorrow = right_now + timedelta(days=1, hours=2)
            # even
            process_event(
                "movie played",
                "",
                "",
                {
                    "event": "$pageview",
                    "timestamp": tomorrow.isoformat(),
                    "properties": {"distinct_id": "asdfasdfasdf", "token": self.team.api_token,},
                },
                self.team.pk,
                right_now.isoformat(),
                None,
            )
            event = get_events()[0]
            difference = abs((tomorrow - event.timestamp).seconds)
            self.assertLess(difference, 1)
        def test_ip_capture(self) -> None: