Sprint Android Sends Same Text 30 Times Why does my HTC Evo 4G send a single text message to the same person 30 or 40 times? Sprint won't help us and it drains my battery and annoys people. Thank-you.
You should use a third party SMS client that generates a log message for every message it sends. These log messages can be read out with alogcat or catlog (or with Androids SDK adb logcat). This way you could prove that the message is only send once from your phone. If the receiver then gets the message multiple times this would proof that the failure is on the carrier (mobile network operator) side. Otherwise, ff the receiver only get's the single message, this would mean the error is within Android's SMS App. Try cleaning the app data and, if this doesn't help, do a factory reset.

Unfortunately I am not sure which third party SMS Apps provide such a log feature. I would start by checking out handcent and goSMS, as these are the 2 alternatives I am aware of.