The double-sms bug has been fixed in Google Voice. It's not available in the Market, yet, but you can extract it from the Android 2.1 rom. Instructions on installing it are on droidipedia here:
Google Voice - Droidipedia
Note: This update works on Droid 2.0/2.01 as well. It is called "Voice" instead of "Google Voice" which looks much better on the icon, as well.
EDIT: Thanks go to jared.gould for posting on Issue 5115 - android - Double SMS sending when using GV App - Project Hosting on Google Code and whoever originally found it if it wasn't him, I just reposted it. :-)