Summary:
Known recipe can be purchased in Hall of Heroes.
This recipe is known, thus it cannot be used. This recipe is bound, thus it cannot be sold. There is no "return to the shop" feature. There is no obvious warning that you know this recipe already.
Basically, you can re-buy known recipes which cannot be used, thus money is wasted.
Clearly this wasn't intended behavior, just a small overlook (originally, recipes were not bound so you could still get your money back by selling recipes, so no sort of limit was put in place originally).
I'm surprised with this kind of approach, as a fellow software engineer I always try to think ahead when I make my software: "what will I change if I make this". If you bind recipes that are already known, you should limit people from re-buying these recipes.
Solution 1:
known recipes should not be on sale in Hall of Heroes.
Solution 2:
when buying a known recipe, knight should receive confirmation popup with a list of max 3 recipes they already know. It should look like this:
Warning! You're trying to buy recipes that you already know!
Known recipes include:
Strike Needle
Calibur
Brandish
and more.
These recipes will be bound do your character, you will be unable to sell them!
Are you sure you want to continue?
In my opinion, Solution 1 is easier to make, but you should make sure its clear that the recipe is known.
PS: I'm not some sort of genius, its just common sense. But I'm shocked when I see this kind of approach. I'm used to seeing it from small developers, but OOO / SEGA... thats just entirely different level! You're not freelancers, try to do a better job! Or someone like me will come and take over.