here's one on eBay, should work on a Duetto too, I believe:
http://cgi.ebay.com/Bertone-GTV-ind...ryZ31350QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
The issue is likely that your contacts are pitted, because of the high currents going through. You should install a relay to prevent this from happening. I bought a brand new switch two years ago, and hardly used it, because I hardly ever drive this car at night. So I have not installed a relay yet, and low and behold, even with this new switch my low beams stopped working. Instead of buying a new switch, you can also try what I did. I is removed it from the steering column, and drilled out the 4 rivets that hold it together. Then you can clean the contact, and bend it a little, so it'll work again. After that, I used two nuts and bolts to put it back together. The two biggest rivets I did not replace, the Allen head bolts go through these holes anyway and will pull the two halves together....I hope this makes sense, you'll probably see what I mean once you get in there. I now have a serviceable switch. Good luck