I'm usually not, wait, make that never in a position where I can't think of a good game idea based of a theme. For some reason, "Duel" is just so open-ended that I can't think of anything good enough for myself. I can always just make a mediocre street fighter game, but my mind is stuck in this perfectionist state where I have to make a "perfect" (or really close to perfect) game.
That being said, the only thing I have done, is brainstorm countless ideas for 10 hours, and set up a base for the game in simple Python Code. I have one game idea that I know I will be able to pull off by the end of this weekend, but its sort of a small-scope entry.
My idea in a nutshell was to make a game where two players (or one player and a CPU) duel against eachother by trying to destroy eachothers castles by using different troops, defenses, and traps.
I also have a cold, so I can't do my usuall plan of waking up at 3AM and working for hours straight. Tonight I'm going to try to get some good rest, so I can have a misery-free and productive day tomorrow.