Intra-forest AD migration: One more pre-requisite to consider

After doing intra-forest migrations for many years, I recently discovered some new pre-requisite which I would like to share in this article. It all started with the following computer migration error: “Error 0x000021C7. The operation failed because SPN value provided for addition/modification is not unique forest-wide.” User migrations are failing as well referencing duplicate UPNs. This is where I started digging.