Symptoms

The error is reported during Exchange Item-Level-Restore (From VSS backup):
System 0x80131500 764 10080096 : Error calling Cmdlet.Mailbox 'User Name' isnt unique on database 'Recovery Database Name'.
at AsigraExchange.CmdLet.RestoreMailBox(String sRDBName, String sSourceMailBox, String sTargetMailbox, String sTargetFolder, String[] sIncludeFolders)
at AsigraExchange.Controller.RestoreMailBox(String sRDBName, String sSourceMailBox, String sTargetMailbox, String sTargetFolder, String[] sIncludeFolders) (CExchangeGranularRestore::RestoreMailbox)

The error is reported during step#2 of granular restore from VSS backup

Cause

The restore process detected duplicate mailbox. Most likely caused by changes in the Exchange database, i.e renamed mailbox that resulted in backup process backing up both (duplicate items)

Workaround

Used the Exchange Management Shell (EMS) to mount the database and then use the GUID instead of 'User Name' to restore the mailbox as follow:
1. Using the DS-Client to execute the 'granular restore - step#1' - download the backup set from DS-System to the Exchange server
2. Use Exchange Management Shell to restore using the following commands:
a. Mount-Database RecoveryDB
b. Identified the GUID of the mailbox with the duplicate ID:
Get-MailboxStatistics -Database "Recovery Mailbox Database"| where-object {$_.DisplayName -like "*UserName*"} | Select MailboxGUID,DisplayName,PrimarySMTPAddress,LastLogged*
* Two GUIDs were returned