If it was frozen by the Sell-By Date and kept frozen, it should be fine. Duck usually comes in a sealed heavy plastic wrapper like a turkey. That protects it from freezer burn.
If you froze it a day before the SB Date then you should cook it right away. Don't leave it in the fridge for days.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan