Go to your user list on the left side Menu:
Click "New user button"
The values "First Name", "Last Name" and email address should be unique:
The system will not allow creating 2 users with the same email address.
If the user cannot be created
It means that there's an existing user with the same details. You should change their information to something else, like "name.surname@test,com".
If the user with such credentials cannot be found, you should check if the user was not "Archived":
If that's the case, you should:
Restore the user;
Make the corrections to credentials (if needed);
Delete such user again;
Create your desired user.