I actually was considering the Unique keyword a long time ago, since I was also inspired by LOTR TCG and it has this mechanics included.
A card with unique keyword could be present only in one instance in your hand. This effects normal drawing, probably special card effects that add cards to your hand would ignore this constraint (like black market for example). However I didn't find this necessary, so the feature wasn't added.
Right now we have the anti-flood protection, that lowers the chance of getting the same cards on hand by half for each card instance. Also now it covers all types of drawing, not just the standard draw. So, still I don't think Unique keyword is needed.
Just one more note to durable cards. Durable cards stays on hand, however this can be also seen as a disadvantage, because you are not getting new cards.