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)
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