I concur with Sharon's answer. Responding with "Duh" is quite insulting as it implies stupidity.

I also agree with Sharon that to a closer friend this may not be rude, but otherwise it would be. Even with friends it can depend on tone of voice.

'Duh' is a very sarcastic equivalent to 'of course', implying that the remark you are responding to was so obvious as to be completely stupid. You might get away with it to a close friend, but to anyone else it could appear fairly rude.