Season duck breasts with salt and pepper. Heat oil in a large skillet over medium high heat. Add duck breasts and brown on one side. Flip over and cook for 1 minute. Add balsamic vinegar and next 3 ingredients. Remove duck breasts when just medium-rare and keep warm. Add rosemary and reduce liquid in pan to about 1/4 cup. Taste liquid and add more vinegar or plum preserves to suit your taste. Too sour? Add more preserves. Too sweet? Add more vinegar.