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1.
Duuh is a typo of "Duh".
Often like "Lool" this is caused by fat finger syndrome (FFS)
Either that or you're just too stupid to type out "Duh"
"Duuh" is also the sound a walrus makes (Or at least we imagine so)
Example 1:

Sarah: OMFG YOU SAID YOU DIDN'T HAVE A MO PED. I TOLD YOU THAT YOU LIE! LIES! ALL OF IT! AND IN ADDITION TO THAT, IT'S SPARKLY BLUE!

Morgan: HAHAHAHA, DUUH

Example 2:

Walrus: DUUH!
by SexyScreamoSerrrrah March 24, 2011