That would be the stock app. Just like using the dialer part of the phone you're using the stock dialer app.I am not using an app. my phone has a messaging system in it already. that is what i am using. I dont know how to turn it off.
Following this post, have the same proboblem double notifications using handcent. I only got handcent because animated pics wouldnt work. They do now with handcent. BUT the stock text is better for me. Can I shut off the handcent text part and use just stock?