Frederick this is the same issue I reported to you back in January. I never found a fix, but have been blocking the duplicate posts via robots.txt with this line Disallow: /*?w3tc_preview=1.
I did delete the plugin for a few weeks to see if that fixed it, but I still have duplicate posts showing up. Luckily it's down to 4 from over 1,000.
I've reinstalled the plugin today, so I'll report back if I see new duplicate posts showing up in webmaster tools.
Considering that 3 of us now have the same issue I'd say there's a bug somewhere in the plugin.