I had that same problem, getting the same message twice, and what you do is go into Notifications and you deselect... let me find the link for you, that will be easier.
Here you go. Direct from the Handcent developer's website (choose "d"):
FAQ
Hope this helps! Now, for me... I have to somehow get "my dedicated camera button" back to activating the camera. Somehow, when I installed the Handcent apps, one of them took possession of that button, and now, when I click it, Handcent wants to create a new message.
Oh, the fun never ends!
(Seconds later: I found it! All I had to do to re-assign dedicated camera button to actually taking photos was disable "Quick Compose"! I love this Handcent application!)