I have the same thing randomly happen to me. A text I send to my girlfriend, she'll receive it multiple times, sometimes even an hour after I originally send it. I'm wondering if it has to do with the strength of the cell signal at the time. I'm beginning to think there's a link the low signal I have at times with the multiple sendings. And when I say low, I mean bouncing in and out of no signal at all to barely one bar on 1x. And I've tried multiple apps for texting, ChompSMS, GoSMS, Handcent, and the stock app. Just a thought though.
What were your signal strengths at the time of sending them?