if i am shopping for a recipe (be it hall of heroes, bazaar, or auction house), i sometimes forget the recipes i currently have. all i want to see, is an asterisk (or some form of indication) next to recipes that i already know. this would make it easier to determine wether i have a recipe or not, instead of opening my recipe menu and searching for it (it becomes very menial).
It already does tell you if you have the recipe learned already. Just hover over the recipe and it should tell you at the top of the popup window whether it's a new recipe or if you have it learned already.