Just a tangential shower thought on gamification. I found myself skipping Duo on weeknights so I could cram on the weekend, get matched with procrastinators, and ascend the leaderboard easier.

I've heard other reports of people focusing on the game and not their goals. Game designers are aware that humans will "optimize the fun away." In the case of Duo, it seems a group of users will optimize the learning away (regardless of the goals or intention)