The
standard answer to this (dual notifications with Handcent) on most Android phones has been to do this:
1. Launch the "messaging" ,click "menu" button on the phone
2. Select "settings" menu item and disable "Notifications"
Sense enabled HTC phones are different because HTC entirely replaces the stock messaging application. According to their manual, you must do the following to turn them off:
On the All messages screen, press MENU and then tap Settings. You’ll
find several groups of settings. Turn off sent and received notifications.
My assumption is that you've done this already. If you have, and you are still receiving 2 notifications, then there's a good chance there's an incompatibility issue between HTC messaging and Handcent. It's happened in the past, and usually Handcent fixes these issues fast. I have interacted with their support group, in the past, and they have always been good: help@handcent.com.
You could also try
GoSMS, which is almost a carbon copy of Handcent, once it's configured properly.
Last, but not least, I will have this phone by 3 today. Handcent is always the first thing I install on every new phone, so I will be checking this out myself.
P.S. -- I would get rid of that task killer, or this won't be the last issue you have with the TB.