Use a knife to score the duck breasts through the skin and fat but not all the way through to the meat.
Heat a large heavy frying pan on medium high. Fry the duck breasts skin side down, until the skin browns and fat is released, about 10 minutes. Remove the breasts from the pan, and pour off most of the fat.
Mix the sea salt, cinnamon and demerara sugar together and sprinkle over the skin of the duck. Return breasts to pan, and fry skin side up for another 10 minutes, or until desired doneness. Turn over once and sprinkle with more of the sugar mixture, and cook 1 minute more. Remove breasts from pan and allow to rest.
Mix together the red wine, crème de cassis and cornflour in a small bowl. Pour into the pan, and simmer for 3 minutes, stirring constantly, until the sauce is thickened. Add raspberries, and simmer for another minute until heated through.
Slice the duck breasts thinly, pour a little sauce over the top, and serve warm.