Can the same email be used on multiple Duos?

I’m new to Duo and found it while looking at ways to protect RDP with 2FA. I created the admin account and then created a user under a different email. I am trying to get my friend to put it on their Remote Desktops as well but want to know if I give them the same email as the one i used for my user will I be able add it or will it cause a conflict?