Why task "promote this server to a domain controller" shows despite the server being already added to a domain?

Solution 1:

You've joined the server to the domain but you haven't made the server a Domain Controller for the domain. Adding the AD DS Role does not make the server a Domain Controller for the domain. If you do not intend for the server to be a Domain Controller for the domain then remove the AD DS role.

If you only want to manage the domain from the server then you only need to add the AD DS Role Administration Tools.

Solution 2:

This also happens if you finish the DC promo using the command line instead of the Wizard. Just hit Promote the Server to a Domain Controller and then cancel the rest. The warning will disappear once it knows the promotion has finished.