This sentence written this way to expressing one's feeling, and there are different ways to do this. You can understand it as if the person writing is adding in feelings in their words, like using different tones when speaking.
From the first part of the sentence, you can tell this approach is self praise style and acting cute whilst doing it. So in this case the person is trying to say "I'm great", but doing it in a cute way. "棒" means great/nice/awesome as @enrico explained already. "棒棒的" is like rather than saying "爸" (Dad) one might say "爸爸" (also Dad). It does not change any mean here. The reason why I am saying it makes the person saying this sound cute is because when kids are younger, they would say "Daddy" and once they are older they will chan say "Dad" instead.
Hope this helped.