Hi fan0258
According to the feedback on this forum, there has only been one success so far back in June 2005.
You shouldn't have to worry too much. There could have been a number of reasons why she hasn't responded to her fan mail yet - e.g. too busy, away filming somewhere etc.
She probably will get on top of her fan mail eventually. When, we don't know.
If you want to send another letter to her again, that's no problem at all. I really do not believe that you have to mention that you have already sent out a letter previously though. Just write it as you would normally write to a celebrity (and include a SASE to make it easier to receive something back.)
When Amanda Bynes does eventually get on top of her fan mail, she'll probably respond to both the letter you're sending now and the one you sent ages ago.
Best of luck
nowandagain [rockon]