Is there a difference between "magret of duck" and "fillet of duck"?
From  Wikipedia  magret refers to a specific breed (the Mulard, not to be confused with the Mallard):


  Magret refers specifically to the breast of a mulard or Barbary duck that has been force fed to produce foie gras.


From  dartagnan.com :


  Sometimes called “duck steak,” the magret (breast) of the Moulard duck is known for its rich flavor and dark red meat. Try it pan seared and find out why the French swear by the duck.