During an Exchange migration you sometimes have the issues where users complain that there are duplicate entries of their calendar items. This is normal to happen and can be done directly from the server using Powershell . You will need to download the script here and then install Managed API 1.2 or later which can be downloaded here.
Remove-DuplicateItems.ps1 [-Mailbox] <String> [[-Type] <String>] [-Server <String>] [-Impersonation] [-DeleteMode <String>] [-Mode <String>][-WhatIf] [-Confirm] [<CommonParameters>]
The syntax is below:
-Mailbox is the name of the mailbox to process;
-Type determines what folders are checked for duplicates. Valid options are Mail, Calendar, Contacts, Tasks, Notes or All (Default);
-Server is the name of the Client Access Server to access for Exchange Web Services. When omitted, the script will attempt to use Autodiscover;
-When the Impersonation switch is specified, impersonation will be used for mailbox access, otherwise the current user context will be used;
-DeleteMode specifies how to remove messages. Possible values are HardDelete (permanently deleted), SoftDelete (use dumpster, default) or MoveToDeletedItems (move to Deleted Items folder).
-Mode determines how items are matched. Options are Quick, which uses PidTagSearchKey and is the default mode, or Full which uses a predefined set of attributes to match items, depending on the item class.
For this to work you need to setup the impersonation of the user which you will be running the script. This can be done as below for Exchange 2010
New-ManagementRoleAssignment -Name ImpersonationRole -Role ApplicationImpersonation -User administrator
-Name : Can be anything you put in, it’s just a name
-User: Is the user you will be impersonating as
On Exchange 2007 you can use the below command:
Get-ExchangeServer | where {$_.IsClientAccessServer -eq $TRUE} | ForEach-Object {Add-ADPermission -Identity $_.distinguishedname -User (Get-User -Identity User1 | select-object).identity -extendedRight ms-Exch-EPI-Impersonation}
More info on the script here