Whenever I click on Invite friends from Facebook, than click Skip (I don't want Duolingo to post on my behalf), and the list of my friends appear.

However, all of the friends who are already on Duolingo are automatically added as my friends, without any notice. The only thing to fix this is to go to my friends list and unfollow them.

I understand that this is to extend every user's network, but hey, that's not the proper way.