To deal with the notification you're receiving from the default messaging app, just go into that app and turn off notifications all together if you're using Handcent. That gets rid of one of them.
You should (note "should", I haven't tried this myself) be able to handle the GV notification issue the following way:
- In your GV settings (online), go to the "Voicemail and SMS" tab, then under the "Voicemail" section, check "Send a text message to:" and your normal mobile number.
- Turn off GV notifications (on the phone app).
Ideally, this would mean that the GV app on your phone will never directly notify you of anything. When you get a text, it will show up in the app, but the only notification you should receive on the phone will be from Handcent, since you already set that up. Finally, when you receive a VM, your GV account should send you a text indicating this, then if you want to hop in the GV app to listen to it, you can, but you should still only be receiving one notification.
The only problem I see with this workaround is that you can't really distinguish between a text and voicemail since they will all be coming through SMS. That may or may not be a big deal for you.
Try it and let me know if it works, I was just screwing around with settings trying to figure out if that actually solves your issue or if I'm just an idiot.