Season the duck breast with salt and pepper. Heat the oil in a large saute pan over medium-high heat. Brown the duck breast, skin side first, then flip over, reduce heat and cook other side, about 10 to 15 minutes. Remove the duck breasts to a platter, reserving the juices in the pan. Work in batches, if necessary.
To make the sauce, add the balsamic vinegar and red wine to the pan and cook until reduced by half. Add the stock and reduce by half again, then add brown sugar, rosemary and berries and cook for 3 minutes. Strain into a bowl and whisk in the butter to finish. Place the duck breast on each plate and drizzle the sauce around.