Its a keyboard abbreviation for "ok" because its quite simply easier to hit "k" 2 times then typing "ok".
I don't care if some danish kids use it to insult people. I have been using "kk" as a confirmation to chat messages for 10 years now, I am not gonna stop just because some people want to see an insult in every letter.
SO next time you threaten to report me for this, please know that I am literally on the floor laughing about how absurd you are. Have a really nice evening.