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
process_event(
                "some-id",
                "",
                "",
                {
                    "event": "$snapshot",
                    "properties": {"$session_id": "abcf-efg", "$snapshot_data": {"timestamp": 123,}},
                },
                self.team.pk,
                now().isoformat(),
                now().isoformat(),
            )
            self.assertEqual(len(get_session_recording_events()), 1)
            self
            event = get_session_recording_events()[0]
            self.assertEqual(event.session_id, "abcf-efg")
            self.assertEqual(event.distinct_id, "some-id")
            self.assertEqual(event.snapshot_data, {"timestamp": 123,})
        def test_distinct_with_anonymous_id(self) -> None: