Can't create user "The data you entered already exist"

Can't create user "The data you entered already exist"

Hi,

I've added some users to my account, but when adding two of them I get "The data you entered already exist", but the list as no reference for any of those users.

example

"julie@mydomain" can't
"julie2@mydomain" I can

Am I missing something here?

thanks,
Francisco