set_once(distinct_id, prop, to, modifiers, callback) {
        const identifiers = {$distinct_id: distinct_id};
        this._set(prop, to, modifiers, callback, {identifiers, set_once: true});
    }
people.set_once(distinct_id, prop, to, modifiers, callback)
        ---
        The same as people.set but in the words of mixpanel:
        mixpanel.people.set_once

        " This method allows you to set a user attribute, only if
            it is not currently set. It can be called multiple times
            safely, so is perfect for storing things like the first date
            you saw a user, or the referrer that brought them to your
            website for the first time. "