If you look carefully on the print menu near the bottom, you should see 'More settings.' If you click that, more settings will appear, one of which is double side printing.

I am sorry David but you are wrong. I have already "More settings" available, and there is no double side printing. I have to say that I use the Swedish version. Could that be the problem. But it worked OK in my old computer a month ago.

That may be the cause. Be sure the print is set to a real printer, as if you're 'printing' to a PDF printer, the option won't display.