1. Bring a pot of salted water to a boil. Add the rapini and cook for 3 minutes or until just tender. Transfer the rapini to a baking sheet and let cool. In the same boiling water, cook the spaghetti, stirring occasionally, for 5 minutes. (The pasta will finish cooking in the Zinfandel). Reserve 1 cup of the pasta water and then drain the pasta and set it aside. Return the empty pasta pot to the stove.
2. Add the wine and sugar to the pasta pot. Bring to a boil over high heat and cook until reduced by half, about 8 to 10 minutes. Add the pasta and shake to prevent the pasta from sticking. Gently stir with tongs until coated and boil over high heat, stirring occasionally, until most of the liquid is absorbed and the pasta is al dente, about 4 to 5 minutes.
3. While the pasta cooks in the wine, heat a large, deep skillet over high heat. Add the olive oil, reduce the heat to medium-low and cook the garlic until golden, about 3 minutes. Add the rapini, red-pepper flakes, salt, and pepper, and cook stirring occasionally, for 2 minutes. Add ½ cup of the reserved pasta water, or more if desired, and stir to combine.
4. Add the rapini mixture to the pasta pot, toss gently, and transfer to a platter. Sprinkle with the cheese and serve.
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