I thinks it's fine to reuse your material, just don't purely copy and paste. That might send the wrong signal (laziness of thought, is that all there is to this applicant?, etc.) I am using my secondary essays as a chance to mention points about myself that I was not able to include in the AADSAS one as well as an opportunity to mention what I find interesting about that particular school and so on. Even if you have already answered 'everything' in your first essay my advice is to put a new twist on some of the answers.