I ran into this question, writ large, in high school, so my perspective may be skewed.
In 7th and 8th grade I took a 2-year course on American History. We used a standard textbook pair, and our tests were the generic ones from the teacher's guides, though as I was in an honors class the answers for matching and multiple choice questions were blanked out and we were expected to supply our own answers. (Our grades were also based on a fair bit of supplemental work.)
I moved to another state between 8th and 9th grade. In my new school, 9th and 10th graders took a two-year course of... American History. From the same textbooks. My teachers administered... the standard tests from the teacher's guide.
Every exam I took in History class during 9th and 10th grade was the exact duplicate, in much easier form, of a test I had taken 2 years before.
I saw zero ethical problems in studying for my tests by reviewing my own graded papers from tests I had taken at a lower academic level.
In your case, you are being assigned for homework problems that you have already done during your own independent learning. If they are well-known problems then the solutions are probably available online, as well. I'd suggest that you attempt to solve them again from scratch, so that the skills and techniques are fresh in your mind (the true underlying goal of most homework), and only refer back to your previous solutions if you get stuck, but I see no ethical issues in applying work that you previously did while satisfying your own intellectual curiosity.