If you are using 'Mesh>Smooth' from the menu at the top or the icon it WILL double up on the faces...its a proper smooth. The other "Smooth" is just a preview smooth...it is set by default in the numbers 1,2,3 for different smooth tolerances. 1 is none..2 is half....3 is total smooth (what it will look like when you use the Mesh>Smooth).

I got the two confused when I first started. If you have changed the 'Mesh>Smooth' to a hotkey and assigned it to number 3 then you would have overwritten the original default.

Hope this helps

cheers bullet