It seems like Bioware didn't think of the situation which may occur when the players complete the quest conditions before accepting it.
That's shockingly easy to do, actually. I'm only surprised there isn't more of it.
In some instances it *is* possible to tell the person the quest is already done. I would swear that, on my first playthrough, I was able to tell Judith that I killed the wyverns already before we met (but I don't think she gave the reward). And I was able to give Frederic his research supplies and the book on draconology immediately because I found them before going to talk to him. But yeah, one of the problems with any multi-component quest is that it's easy to complete it out of sequence.
My big complaint is that you get requisitions after the materials for it are no longer available. You get Decorative Gems and Fereldan Locks from the warring templars and apostates in the Hinterlands, but you can get the Inquisition Tapestries and Prisoner Cages requisitions at any time. That leaves you with open requisitions and no way to fill them. I wish there was a way to say "I'd like to help, but the resources to fulfil this requisition are depleted." Ditto with the Dwarven History requisition in the Hissing Wastes. You get the statuettes and plates and assorted junk from containers, not drops, as a rule, so once you've looted, pillaged, and presumably sold articles you didn't think you needed, the requisitions go unfilled. Also, it would be nice to have the ability to say "I'll pass on that one. Do you have anything else?"