Can I substitute duck breast for leg?
Like poultry, duck has fatter legs than breasts and also the meat itself is darker, adding a different flavor to the meal.
The fat in the legs will melt and the potatoes and onions will cook in it.

It would be great if you had some duck fat to add to the breasts or at least any kind of fat, for the flavor.
But if you like to eat learn unfatty meals, you can stick with the breast only.