2 of my students have existing Duolingo accounts so they can't join my class when using link
They should be able to link their existing account to your classroom by clicking your link after logging on their existing account.
Tell them to first log into their existing account, then to click on the link.
However, if you and them really want to create new accounts with the same email than the existing ones. Do the following:
- in the existing account, change the email(*)
- Change also the username(*) if they want to use the same in the new account (in which case, better to just link their existing account to the classe and not do all this process)
- Log out
- Click the link of the classroom you shared to them and create a new account with the email address (and the username if step 2. done)
(*) You have to replace the email by something else. If they put a non-valid email and change also the username: they'll have to remember what they put as username in their existing account if they ever want to access it again.