back at it (again)
Having resumed doing something, often something negative or unpleasant. A: "Do you hear arguing?" B: "Ugh, those two must be back at it again." I tried to stop biting my nails, but I'm back at it already. A: "Bill still isn't here?" B: "Nope. Even though the boss has warned him about being late for work, it seems he's back at it."
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