Which code are you talking about? There is the pre-order code, which you entered when you first reserved your copy of the game. Then there is the product registration code, which you get with the boxed version of the game or in an email today or this weekend from Origin.
If you are trying to re-enter the pre-order code, then the error is expected: you've already entered that code. If, however, you are trying to enter your retail product key (CD key) and are getting that error while your account says you haven't entered the retail product key, you should post in this thread