Score duck through the skin and fat each way on the diagonal in a cross pattern. Season with salt & pepper and rub salt & pepper into each breast. Leave duck breast on the side, (skin-side up), at room temperature, for 15-30 minutes.
Whisk chicken broth, Grand Marnier, sherry vinegar, orange marmalade, and orange zest, together in a bowl.
Heat the duck fat in a heavy skillet over a medium flame for 2 minutes. Place the duck breasts in a skillet, the skin-side down, and cook for 6-8 minutes. Turn over the duck breasts and cook until they start to firm and are reddish-pink and juicy in the center, about 3-4 minutes longer. Pour any remaining duck fat into a small bowl.
Return the skillet to medium heat and whisk the flour into pan; cook and stir until the flour is completely mixed, about 1 minute. Pour the orange mixture into the skillet; bring this to a boil. Cook until the sauce is thick and is reduced, about 3 to 5 minutes. Reduce flame to low. When the orange mixture stops bubbling, add the butter; stir until butter is completely melted and incorporated into the sauce, about 1 minute. Season with salt to taste.
Slice duck breasts across the grain and spoon orange sauce over the slices.
