"Ah-Duh" is a combination of the "Ah-Hah" moment of realization and the "Duh!" moment of having realized something that should have already been obvious. Ah-Duh therefore captures the feeling one has when one excitedly realizes the obvious and/or believes they've had sudden insight but then realizes they have "re-realized" something they already knew to be the case.
Wow, I just realized I can plug my iPhone into my axillary audio jack in my car and enjoy only the audio of online lectures and debates on Youtube. I normally think of Youtube has being only valuable for video! Ah-Duh!