How to use the socketshark.constants.ERR_ALREADY_SUBSCRIBED function in socketshark

})
        assert client.log.pop() == {
            'event': 'subscribe',
            'subscription': 'simple.topic',
            'status': 'ok',
        }

        await session.on_client_event({
            'event': 'subscribe',
            'subscription': 'simple.topic',
        })
        assert client.log.pop() == {
            'event': 'subscribe',
            'subscription': 'simple.topic',
            'status': 'error',
            'error': c.ERR_ALREADY_SUBSCRIBED,
        }

        await session.on_client_event({
            'event': 'message',
            'subscription': 'simple.invalid',
            'data': {'foo': 'bar'},
        })
        assert client.log.pop() == {
            'status': 'error',
            'subscription': 'simple.invalid',
            'event': 'message',
            'error': c.ERR_SUBSCRIPTION_NOT_FOUND,
        }

        await session.on_client_event({
            'event': 'unsubscribe',

async def subscribe(self, event):
        """
        Subscribes to the subscription.
        """
        require_authentication = self.service_config.get(
            'require_authentication', True)

        if require_authentication and not self.session.auth_info:
            raise EventError(c.ERR_AUTH_REQUIRED)

        if self.name in self.session.subscriptions:
            raise EventError(c.ERR_ALREADY_SUBSCRIBED)

        await self.authorize_subscription()

        await self.shark.service_receiver.add_provisional_subscription(
            self.session, self.name)

        result = await self.before_subscribe()

        self.session.subscriptions[self.name] = self

        if self.should_deliver_message(result):
            await event.send_ok(result.get('data'))

        await self.shark.service_receiver.confirm_subscription(
            self.session, self.name)