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.team.anonymize_ips = True
            self.team.save()
            user = self._create_user("tim")
            Person.objects.create(team=self.team, distinct_ids=["asdfasdfasdf"])
            process_event(
                "asdfasdfasdf",
                "11.12.13.14",
                "",
                {"event": "$pageview", "properties": {"distinct_id": "asdfasdfasdf", "token": self.team.api_token,},},
                self.team.pk,
                now().isoformat(),
                now().isoformat(),
            )
            event = get_events()[0]
            self.assertNotIn("$ip", event.properties.keys())
        def test_alias(self) -> None: