First used to make fun of a typo made by a secretary for a private school in Kansas. The letter "d" was accidentally inserted into a student's name that ended with a "t" making the name end in "dt". This sparked the notion to pronounce these two characters as "duhtuh" and add it to words, phrases, and sentences.
Aaron: Wtf, no one cares duhtuh.
Jon: Hahaha duhtuh....
Carrie: So, did you enjoy the movie, Doug?
Doug: I hated it duhtuh.
Carrie: Haha, me too duhtuh.