I was fiddling around with the settings and think I found a workaround. If you go into Settings --> Applications --> All --> Messaging --> Disable
You will get a warning that you are disabling a built in service. Yes you want to..
This disables the build en SMS service completely.
This seemed to work for a little while. But then started sending duplicates again
perhaps a clue?
Model: Swedish, I9100XXLPQ, 4.0.3