Symptoms
Duplicated ReadFlagChange items are returned in a SyncFolderItems operation response in Microsoft Exchange Server 2019 and Exchange Server 2016.
Cause
The SavedReads and the Reads downloaded in current session are both processed again. It leads to duplicated items in the response from the server.
Resolution
To fix this issue, install one of the following updates:
For Exchange Server 2019, install the Cumulative Update 2 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019.
For Exchange Server 2016, install the Cumulative Update 13 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
References
Learn about the terminology that Microsoft uses to describe software updates.