I have the same issue. I called Sprint Tech Support and they couldn't figure it out. They checked my texts from other Sprint customers that had duplicate texts and found that some of the texts were buffering or still waiting to be delivered. I am assuming that when the texts gets sent to my phone, my phone is not communicating back to let the network know I received the text.
They eventually told me to go to the store. The store told me that it could be an issue with an app. I remember distinctly having an issue with Google Voice when I was setting it up. The whole GV set up is a bit confusing. Anyway, I had them do a hard reset to wipe out my phone. I won't download GV to see if that was the issue. I've since received texts that I received before the reset. I am hoping that those texts won't pop up again, meaning, while it was in buffering status, my phone actually communicated back to the network as transfer complete.
I've installed, uninstalled, then installed GV before the reset. I think you really have to do a hard reset to resolve this issue.
Do you have Google Voice installed?