Attending lectures for a course I already passed?
At least I would not discourage anyone from sitting through a course again. The notion that one has permanent and total recall of almost anything is untenable: one must review and rethink (or use) periodically, or things fade away from disuse. For that matter, there are usually several layers of understanding/appreciation, and getting to the subtler ones takes time and (re-)thought.

Perhaps you'll be better off than others who got their grades and don't even realize that they'd benefit from re-thinking the material.