The core leadership deck has two copies of Steward of Gondor. The only way you'd get to play the second one is if you have the first attached to a hero who dies, right? So what's the rationale for including a duplicate? Just to make it more likely you'll draw it at least once since it's so useful?

Ok, I get what your asking but think about this. if you have two copies of any card, it is twice as likely to come out early. also, in some quests, you have to discard attachments or even cards. It is just such a good card, arguably THE BEST CARD, that everyone wants to get their hands on it as quick as possible.

In other words, "yes". Whether a card is unique or not, you generally judge the number of copies you put in your deck by how consistetly you want it to show up, and that is also affected by how much card draw you include.

What has already been said. I have unique attachments and allies that are so good I'll consistently include 2-3 copies of them so that they'll hopefully show up early in the game.

In other words, if you have the chance to include even 3 copies of the Steward in your deck, grab it. It's that good.