ADPREP Error – Promoting Windows Server 2016 in 2008 R2 Forest/domain

When promoting a Windows Server 2016 to DC, adprep fails with an error that an attribute or value already exists. I am unsure if I caused this by attemping to add mutiple DC’s at the same time (race condition).

Error: The DN is CN=Send-As,CN=Extended-Rights,CN=Configuration,DC=.

The error logs were located: C:\Windows\debug\adprep\logs\

Entry DN:…