I have eaten a duck dish at Chefs de France, which was horrible and had obviously been sitting under a heat lamp, and at the Hollywood Brown Derby, which was sooooooooo delicious.
I also remember seeing duck on the menus at Yak & Yeti, Bistro, and Jiko's. It's an appetizer on some menus as well (e.g., Citricos). There was also an appetizer on Le Cellier's dinner menu, but I don't know if it'll still be there after the switch to Signature.
I've had duck at the California Grill, the Kona Café and Yak and Yeti. All three were appetizers of some sort. I can't remember for sure whether I've had duck anywhere else there. It's nearly a guarantee that if duck is on the menu, either I or my boyfriend will order it.
If you click on the word DINING on the line directly above the <POST REPLY> button at the top of the page it will take you to the WDW Info Dining Section. Put "Duck" in the search box. It will give you about ten results, with descriptions.