Hey just joined to answer this question :D
Hey sometimes Android firmwares give this problem and just reinstalling your phone's software fix it. Try reinstalling Android with Samsung kies.
But if that doesn't solve it, it may be a problem with the hardware.
My friend has the same problem and I fixed it with just a scavenger hack.
Maybe your problem is different but I think you should give my hack a try.
So here goes.
The problem maybe with your button itself. I mean the very hardware itself, Samsung phones generally don't have this issue but Duos on the other hand was not that fortunate.
So You may wanna show it to a local vendor if you are not sure about opening you phone and try it yourself. Ask the repair man to check and see if they can short the PCB connections of the button and get a response from it. If you do see a response from the phone then ask them to change the button and that will just magically solve the problem.
If you want to open your phone itself then you can get that button out of an old phone as almost any button from any phone's KEYPAD will work.
But remember that buttons from keypads are generally just small beveled metal plates that may look this.
[image|314751|align=left]
The silver metal round plates are the buttons that I am referring to.
Good luck and god speed :D