I don't think it will work if you already own it (from an earlier free deal in my case). I tried to use the coupon while not logged on and it works; but as soon as I logged, it stopped the transaction saying I already owned the game (and could download again it from my account or some such) but it's good news for others.
I went back to the older Hallmark type promotions and used one offer for the second time, only choosing a game (Majong Towers of Eternity) that I had passed on the first time. It counted as as a 'Fest purchase which was just what I needed it for.
I hope they do a Decemberfest because with the two free ones from November and the Daily Deals, I'm buying some discounted games I like throughout the month in any case.