Active Directory schema extension issue if you use a Windows Server 2025 schema master role

We have this Problem in our Environment. I paid for an urgent Ticket, set the Severity to A - Urgent, but after 55 hours now i still didnt get a response. Calling Support doesnt help at all. They all say they are going to escalate that, but still no answer. I found the duplicates, but when i try to remove the duplicates via ADSIEdit, the duplicated entry comes back after opening the Object again. This is serious, and i dont know hot to solve this problem by myself.

The Workaround

While Microsoft hasn’t yet released an official hotfix or KB article as of October 2025, here’s what worked in this environment:

1- Isolate the schema master role (FSMO) on a Windows Server 2025 DC.
2- Export the schema using ldifde -f schema.ldf -d "CN=Schema,CN=Configuration,DC=...".
3- Manually locate duplicate attributes (often Exchange-related, such as msExchMailboxGuid or msExchAttributeSet).
4- Compare attribute OIDs and GUIDs—retain the newer SE versions.
5- Clean orphaned or duplicated entries via adsiedit.msc or ldifde imports (extreme caution required).
6- Force replication again using: repadmin /syncall /A /e /P
7- Once replication stabilizes, demote the older DCs and complete the migration.

Hi,

First, thank you for informing us. Sadly enough, this is highly inconvenient for us. Recently we introduced for many of our Customers 2025 DC's because we had to deal with strong certificate mapping. 
Thinking about it though, it generally should not be too much of an issue to temporarily borrow another server to promote it to DC and do this.
Still, it would be much appreciated if we could be provided publicly with the script support has to clean up duplicates in the schema. It would save me (and others) the hassle should we encounter the issue. 

Well... i've upgrade to Exchange SE and later upgraded DC's to 2025.
I'm not running into AD replication issues (yet), but i'm stuck because AD and Forest functional levels were upgrade to 2025.
What happens if SE CU1 comes out before Windows fix?

We are migrating from a 2019CU15/WS2022 to SE/WS2025 on a "WS2025 only" domain (ie. all DCs are 2025), and we are not having this issue. When SE was being installed we were simultaneously in the process of removing the last WS2022 DC and we are not sure if SE was installed before or after the FMSO role was transfered to a 2025 server but in any case we are not seeing any replication errors in the logs. If all seems to be OK, is there are a chance they may show up in the future or are we in the clear?

Nino_Bilic​, what is the recommendation if all DCs are 2025 and we are working toward transitioning to SE given that EOL support is 14 Oct 2025?  Do we just proceed with the transition and open a ticket with support?  Or will there be another path like extending 2019 support?