The double j/k is a sometimes annoying practice of saying jk/j/k multiple times in a row. It is thus an example of epiphora, for you linguistic nerds out there. Ordinarily used when messing with someone. Occasionally double j/k's are used to make someone THINK you were joking.
Prima: I banged your sister.
Prima: also I crashed your car.
Secunda: ARE YOU FUCKING--
Prima: double j/k! *cough*
Prima: I hate you.
Secunda: ..you ass.
Prima: YOU'RE an ass, God, you're such a fuckhead sometimes! I can't believe you!
Prima: J/K! Hahahaha!