I've seen other people with this problem and it was really bugging the heck out of me. The problem was that I was getting dual notifications for SMS when using Handcent or ChompSMS. I made sure to go into the default messaging app and turn off notifications, but it was still doing it. Very annoying! I've seen other people with the same problem. I finally figured it out. I had downloaded Ringo Lite a long time ago, which allows you to do a little more customization of ring tones for different contacts. I had totally forgotten about it. It had the default sound set as my SMS tone, so when an text would come in, Ringo Lite would play the default sound and Handcent would play the sound I had configured. I removed Ringo Lite and all is as you would expect it to be. So, if you are getting multiple notifications and you can't figure out why, go through your apps and see if you have a different app that is playing that notification.