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
Person.objects.create(team=self.team, distinct_ids=["anonymous_id"])
            Person.objects.create(
                team=self.team, distinct_ids=["new_distinct_id"], properties={"email": "someone@gmail.com"},
            )
            process_event(
                "new_distinct_id",
                "",
                "",
                {
                    "event": "$identify",
                    "properties": {
                        "$anon_distinct_id": "anonymous_id",
                        "token": self.team.api_token,
                        "distinct_id": "new_distinct_id",
                    },
                },
                self.team.pk,
                now().isoformat(),
                now().isoformat(),
            )
            # self
            person = Person.objects.get()
            self.assertEqual(person.distinct_ids, ["anonymous_id", "new_distinct_id"])
            self.assertEqual(person.properties["email"], "someone@gmail.com")
        def test_distinct_with_multiple_anonymous_ids_which_were_already_created(self,) -> None: