The rarity works in case you use the booster plugin also o/ Which is this one:
https://raizen884.itch.io/ignis-collectible-cards
I think it was 10, I specified on the booster plugin help, If I'm not mistaken I did rarity from 1-10.
"And is it possible that an enemy has a unique card? " Yes it is :), all enemies have "reserve cards" which would be the cards that fills his hand in case he goes below 5 cards (after you win some of them). You just have to add the unique card to their hand, but not add to their reserve cards, that would make it not have more than one in the game.