Each skill kinda requires you to get 5 instances of it, simply because if you get a skill in your deck, you want it to be as powerful as possible, thus it limits the amount of cards that you can use.
Edit: Another reason that I dislike this mechanic is that it forces you to discover 5 instances of the same move. While discovering a single skill is way better than it used to be, since you go out, you kill animal, get lp, if you have a bit of luck you get a new, cool move, and you are happy. Discovering new skills is fun - getting ones that you already have is not so interesting.