Well, yeah, it was the most obvious plot twist out there as evidenced by the fact most of us figured it out nearly right away. But that does not mean the same thing as the writers intended us to figure it out right away. I'll entertain that as the episodes were being written they may have begun to realize the audience might clue in earlier than intended, which led to them changing the nature of Ash/Voq's transformation. EG instead of a simple surgical alteration and transplanted memories he now becomes some sort of weird surgical merging of the two people because no one would have expected that. Which we wouldn't because it doesn't make any damn sense.
However the writers most certainly did not sit down as they were beginning the season and say "we're going to have this one officer actually turn to be a Klingon sleeper agent and we want to make this painfully obvious to the audience as soon as he's introduced before we make the revelation official half a dozen episode later."