Students already have Duolingo?
What if students already have an account before I start my class? Am I still able to add them and they can keep their progress, or do they have to start all over?
De nada. :)
Yes, you would still just send them the link. The process to add them is the same as adding any other student, no specific extra instructions are necessary.
They can use the link and then log in, or they can log in and then use the link, in either instance they will land on their Home page with a pop-up message asking them to confirm that they would like to allow you to monitor them.