Assuming that all game content includes quest rewards this is already covered. I see two reasons for you to receive a known blueprint:
- The game rerolls a finite number of times. If you know most blueprints, then there is a significant chance not to receive something new.
- I understand these quests have a narrow pool - e.g. only blueprints for level 35 relics. If you have all of them, these quests cannot provide you with anything new.
Quest rewards differ from other loot because duplicates are not hidden. Similar to how you keep getting extra bags from certain quests, although you have already unlocked the final fifth one. So, you can trade known blueprints with other players or sell them for some iron coins.