S3 text and Messaging

Hi - I've been having an issue in relation to multiple threads for the same phone number in Handcent. If I text direct from the S3 contacts, selecting Handcent as the app, it creates a separate thread for the message in handcent. However the response will come into the normal handcent thread. So I end up with outgoing text in one thread and the response in another thread. The phone numbers are the same on both threads (fully qualified with + country code etc.).

It only happens if I text direct from contacts on the phone, as opposed to texting direct from handcent. The stock messenger app is the same - 2 threads.

Any ideas ?