I have set up my email addresses using Apple mail problem, that I an account could add, because the error message always came, that the Account should already exist (even though he of course did not).
My iCloud account was set as my same email address “already exists” E-Mail Adresse: .
I've added address email in Apple mail to your iCloud account , Thus there was no duplicate E-Mail addresses and I could my Use address, without getting the error message during Setup.
Instructions for Apple Mail
- Mail (top right) -> Accounts
- Select iCloud account links
- Click on mail in the box to the right
- email@example.com find address and confirm
Then the email can be added to mailbox without the obstructive error message.