Every once in a while you will find customers running in to the following issue. When they try to get mailbox folder permissions or set mailbox folder permissions  a red error message pops up stating: The security principal specified is already on the permission set. + CategoryInfo          : NotSpecified: (:) [Get-MailboxFolderPermission], CorruptDataException + FullyQualifiedErrorId : [Server=SU8000006141,RequestId=628b5335-6148-4297-8f06-4c607b029737,TimeStamp=15-2-2017 06:46:24] [FailureCategory=Cmdlet-CorruptDataException] BE7556A,Microsoft.Exchange.Management.StoreTasks.GetMailboxFolderPermission + PSComputerName       …