I would like something that notifies you if you already have learned a recipe you are attempting to buy or you already own one. A simple thing like this would prevent players from accidentally buying recipes in the HOH (to be forced to sell back for a 1% refund) or just not have to scroll through a long list of recipes you have learned.
As with other notifications you can choose to disable it so that player recipe vendors won't have to deal with it every time.
Don't click this v
My reaction when I buy a 25k 5 star recipe in the HOH to find out I already know it
In terms of knowing a recipe, it says at the top either "known recipe" or "new recipe (click to learn!)". That's covered in the tooltip.
However it would be good to have a warning message that says "Item in inventory (are you sure you want to purchase a duplicate?)" with a checkbox for "don't show me this message again".