I initialized a new Strapi server and with different databases and when registering a new user I keep getting the same "message": "Email is already taken" error. I used a different username and different email each time. I also couldn't get the email confirmation to work at all.
I had this exact issue, here is how I solved it. You have to insure that you give Strapi native email plugin a VALID email address.
Strapi has a built in email plugin that manages emails. You can also use a bunch of 3rd party email services to better manage your email.
Optional Strapi Email Plugins