For various reasons, there are times when Active Directory may encounter a situation where multiple objects appear. The duplicates will be named with a CNF:[GUID] on the end of the name.
To resolve this issue, like you said, we could remove the duplicate object in ADSIEDIT. Note that please backup the AD first before performing the operation.
In addition, you could also check if any related error logs recorded in event viewer which lead to this issue.
Below is a thread discussed about the similar issue for your reference: Duplicate Server Name in DAG Database Management
How's your migration going now? Here is a link introduces about the migration of dag in exchange server, you could refer to the steps introduced in it.(Also applicable to exchange 2019)
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
1) Create the new Exchange 2019 DAG and add Exchange 2019 mailbox servers to the DAG
2) Move mailboxes from databases in the old Exchange 2019 DAG to the new Exchange 2019 DAG
3) Create mailbox database copies within the new Exchange 2019 DAG
4) Remove the database and its copies from the old Exchange 2019 DAG
5) Remove the old Exchange 2019 DAG members and the DAG itself – either in GUI or by running Remove-DatabaseAvailabilityGroup cmdlet
6) Reconfigure DNS, Autodiscover …
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