I'm having a similar problem. When someone sends me a text to my GV number, I get that text but it appears that I also get one in the messages application. I think the solution is in this thread but I just want to be clear.
In GV web settings, I have went to my phone settings to edit them. I have unchecked the box for SMS forwarding. The explanation isn't very clear, but I think that checking this box causes a text sent to your GV number to be forwarded to your regular phone number, correct? So unchecking the box should allow a text sent to your GV number to go to the GV app on your phone, but will not send a duplicate text on to your regular number, correct?
Hope this is right or I'm back at square one. I don't text a lot but getting dinged two dimes each time isn't fun.