You have many Domain Controllers, and you want to prepare for quick replacement in case of a complete failure.
You are using the DCPROMO unattended option "AllowDomainControllerReinstall=1", so the promotion re-uses existing AD objects during the promotion.
You find that the promotion fails. In the DCPROMOUI.LOG you find:
dcpromoui 5C8.BA0 075E 12:02:29.856 OperationStatus : 0x4E2 !0 => error
dcpromoui 5C8.BA0 075F 12:02:29.856 DisplayString : Active Directory Domain Services could not determine if this directory server name CN=NTDS Settings,CN=DC2,CN=Servers,CN=Default-First-Site-Name,CN=Sites,CN=Configuration,DC=contoso,DC=com is unique on the remote directory server DC1.contoso.com. If this name is not unique, rename this directory server.
The error is ERROR_DOMAIN_CONTROLLER_EXISTS.
Only after this it reads the option from the answer file:
dcpromoui 5C8.BA0 0780 12:02:31.200 Enter GetAnswerFileOption AllowDomainControllerReinstall
dcpromoui 5C8.BA0 0781 12:02:31.200 Enter AnswerFile::GetOption AllowDomainControllerReinstall
dcpromoui 5C8.BA0 0782 12:02:31.200 using value from cmdline
dcpromoui 5C8.BA0 0783 12:02:31.200 Yes
dcpromoui 5C8.BA0 0784 12:02:31.200 Enter State::SetDomainControllerReinstallFlag true