But the original owner still wants to receive push, so you shouldn't unregister. This isn't apple's problem, this is that the twitter app's server doesn't know they logged out and continues to send messages to the phone.
If you logged out of the application, it should unregister. It should only re-register when you log back in (which would require communication with Twitter's servers, which would update the APNS token to point to the correct account). This is how I handle APNS tokens in my Pushover app.