"Error code: '-2147467259'. Status: 'Failed'. Message: Error: MapiExceptionDuplicateObject: Unable to open message store" is logged by the mail target agent during a mailbox move operation.
The mail collection is configured to synchronize mail to a different mailbox database than the one used by the Directory Synchronization Agent to mail enable the target AD objects. The mail target agent, when it attempts to move the mailbox to the destination mailbox database, encounters a problem executing the move request and logs the "MapiExceptionDuplicateObject" error.
This error will be corrected in Quest Migration Manager 8.9. To work around this issue with Quest Migration Manager 8.8 and earlier, implement one of the following:
1. When the error occurs, stop the mail target agent, manually move the mailbox to the proper mailbox database using the Exchange Management Console or the Exchange Management Shell, and start the mail target agent. The target agent will locate the mailbox on the new mail store and will start to restore mail.
2. Let the mail target agent continue to execute the mailbox move request. After a number of sessions the agent's move request will complete and the agent will start to restore mail.