Comment With an important caveat! (Score 5, Informative) 201
"As I said earlier, there’s one exception to rule, and that’s DCs themselves. Every Domain has a unique Domain SID that’s randomly generated by Domain setup, and all machine SIDs for the Domain’s DCs match the Domain SID. So in some sense, that’s a case where machine SIDs do get referenced by other computers. That means that Domain member computers cannot have the same machine SID as that of the DCs and therefore Domain. However, like member computers, each DC also has a computer account in the Domain, and that’s the identity they have when they authenticate to remote systems. All accounts in a Domain, including computers, users and security groups, have SIDs that are based on the Domain SID in the same way local account SIDs are based on the machine SID, but the two are unrelated."
The low ramifications of this as mentioned above may have changed post Win2K and XP. This particular caveat governed our processes as system deployment specialists for Microsoft corporate events. We had to make sure that any potential DC had a unique SID even before the machines were promoted to DC, otherwise we saw (verifiably!) many issues with Workstations failing to join the domain. I seem to recall other more esoteric issues with older Microsoft server products, but that may be delusions based on the mass hysteria we had about unique SIDs at the time.