If is says to send it back within 10 days be sure to get in back within 9 days. Of all the secondaries I received none of them gave a time requirement, however, I think it is in your best interest to send all secondaries back as quickly as possible. Early application is the name of the game. The sooner they have your complete app, the sooner they can invite you for an interview, and the sooner they can accept you.
As for what to write on the secondary, I always tried to include more, or at least elaborate more than what was on my AADSAS application. It seems to me that they are not re-asking these questions just out of conincidence. Good luck to you.