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("tim")
            Person.objects.create(team=self.team, distinct_ids=["asdfasdfasdf"])
            right_now = now()
            tomorrow = right_now + timedelta(days=1, hours=2)
            tomorrow_sent_at = right_now + timedelta(days=1, hours=2, minutes=10)
            # remo
            tomorrow = tomorrow.replace(tzinfo=None)
            tomorrow_sent_at = tomorrow_sent_at.replace(tzinfo=None)
            # 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(),
                tomorrow_sent_at.isoformat(),
            )
            event = get_events()[0]
            event_seconds_before_now = (right_now - event.timestamp).seconds
            # asse
            self.assertGreater(event_seconds_before_now, 590)
            self.assertLess(event_seconds_before_now, 610)
        def test_capture_no_sent_at(self) -> None: