Use a 3D duck pan like this one: http://cgi.ebay.com/3D-RUBBER-DUCKY-CAKE-PAN-DUCK-CAKE-TIN-MOLD-BONUS_W0QQitemZ260126970361QQcmdZViewItem
If you don't want to purchase the 3D duck pan you could always carve the duck. Use a mini ball pan for the head and attach a fondant beak. The body you could carve from either round pans or oval pans would even take a little less carving. Stack them together (without filling) and carve into shape. Then take apart and do your buttercream dam and filling. I've never done one of these in fondant, but I personally would cover the head separate and then stack it onto the body using a dowl rod for support all the way through the head and body. (Even if you use the 3D pan, I would dowl the head for support). Then, I would would probably either make a yellow or orange collar to place around the neck to hide the seams.
That's how I would do it, but I'm sure others with more experience can give you more ideas!!!
Valerie
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