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
@patch("posthog.tasks.process_event.posthoganalytics.capture")
        def test_capture_first_team_event(self, mock: Any) -> None:
            """
            Assert that we report to posthoganalytics the first event ingested by a team.
            """
            organization, team, user = User.objects.bootstrap(
                "Test", "testuser@posthog.com", None, team_fields={"api_token": 456}
            )
            process_event(
                2,
                "",
                "",
                {
                    "event": "$autocapture",
                    "properties": {
                        "distinct_id": 1,
                        "token": team.api_token,
                        "$elements": [
                            {"tag_name": "a", "nth_child": 1, "nth_of_type": 2, "attr__class": "btn btn-sm",},
                        ],
                    },
                },
                team.pk,
                now().isoformat(),
                now().isoformat(),
            )
            mock.assert_called_once_with(user.distinct_id, "first team event ingested", {"team": str(team.uuid)})
            team.refresh_from_db()
            self.assertEqual(team.ingested_event, True)
        def test_snapshot_event_stored_as_session_recording_event(self) -> None: