In our environment we have more than 300 devices, these are almost all Windows Servers.
And this single server appears every day as a new device. 7 days, 7 devices, and all with the same GUID.
I noticed that a few weeks ago, I read lots of forum entries, almost all of the SCCM client logs of this server, couldn't find anything really suspicous.
I reinstalled the client twice, for the second time according to this post (https://social.technet.microsoft.com/Forums/en-US/32264a03-1759-4a25-9b21-413c8de9fe4f/client-keeps-assigning-the-same-guid), I thought maybe the Citrix causes some trouble. (We have many other Servers with Citrix, no problems)
So I removed the SCCM client, after a reboot ran the Active Directory System Discovery (that runs automatically only weekly), that found the Server, I approved it (we have no automatic approval), I installed the client from the Console. I checked the machine, the client was there, running.
Not everything seemed right though. In the general informations of the device in the console the Client Check Result was None and did not become Active.
There is a planned restart every workday at 2:00, because of Citrix.
And the next day, right after the restart the new device appears, as I see it is created by the MP_ClientRegistration.
Looks pretty much the same, but one has to approve it (no automatic approval), with the exception, that this one is Active, Client Check Result: Passed, the icon checked with the green mark, the former device gets a grey x.
Actually this new instance looks better, only the AD System Discovery agent is missing, beause that runs only weekly.
But it lasts only one day.