Hi Frogworth - I downgraded back to Fusion 5.x because of these issues. What I did was use AppCleaner to remove Fusion 6.0 (making sure it didn't remove any VM's), then re-install Fusion 5.0.3 using my original key. In my case I mostly use my bootcamp partition, so I had to re-create the bootcamp VM using the wizard in Fusion 5. The bootcamp wizard did try to install VMware tools into my bootcamp partition and failed saying "newer version already installed", which makes sense, but I haven't had any problems with the Fusion 6 VMware tools in my bootcamp VM - it all works just the same as before.
And I'm very happy to say that back on Fusion 5.x the Duet works no problems 