Yes, I had the same problem and MisterEff is probably right. You just have to log into Google Voice on a computer and change the setting where it says to forward SMS's to your mobile device. I do it this way not only to avoid double SMS's... but also because I only have 250 messages a month... and Google Voice texts which are not forwarded to you do not count against your monthly amount.
There is one problem to be carefull about though! Google Voice by default only check the servers about every 15 minutes. For me most of the time this is fine.. but for some people who communicate via SMS a lot... waiting up to 15 minutes between messages can be annoying. You can set Google Voice to sync more often... but then I'd imagine that will start to kill your battery because you will be using more data.
I just left it at 15 minutes. I will use "normal" SMS if I need to get a response quickly. (Or tap the refresh button a few times

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