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(
                "new_distinct_id",
                "",
                "",
                {
                    "event": "$autocapture",
                    "properties": {
                        "distinct_id": "new_distinct_id",
                        "token": self.team.api_token,
                        "$elements": [
                            {
                                "tag_name": "a",
                                "$el_text": "a" * 2050,
                                "attr__href": "a" * 2050,
                                "nth_child": 1,
                                "nth_of_type": 2,
                                "attr__class": "btn btn-sm",
                            },
                        ],
                    },
                },
                self.team.pk,
                now().isoformat(),
                now().isoformat(),
            )
            event = get_events()[0]
            element = get_elements(event.id)[0]
            self.assertEqual(len(element.href), 2048)
            self.assertEqual(len(element.text), 400)