The majority of the time, I don't/can't straight copy and reuse panels, BUT WHEN I CAN I'll do it. No use doing more work when making comics is already so time-consuming. Like these two panels are several pages apart, but it's literally the same panel xD
These are different chapters but the action was the same, to I copied the lineart, lol and only changed it to fit some minor style changes
Reusing backgrounds is the easiest imo. I know I have more examples of me copying the bg, but I don't remember which panels are copied atm.