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.assertListEqual(self.team.event_properties_numerical, [])
            FeatureFlag.objects.create(team=self.team, created_by=self.user, key="test-ff", rollout_percentage=100)
            with self.assertNumQueries(17):
                process_event(
                    "xxx",
                    "",
                    "",
                    {"event": "purchase", "properties": {"$lib": "web"},},
                    self.team.pk,
                    now().isoformat(),
                    now().isoformat(),
                )
            self.assertEqual(get_events()[0].properties["$active_feature_flags"], ["test-ff"])
        def test_event_name_dict_json(self) -> None:
            process_event(
                "xxx",
                "",
                "",
                {"event": {"event name": "as object"}, "properties": {"price": 299.99, "name": "AirPods Pro"},},
                self.team.pk,
                now().isoformat(),
                now().isoformat(),
            )
            event = get_events()[0]
            self.assertEqual(event.event, '{"event name": "as object"}')
        def test_event_name_list_json(self) -> None: