UPN Issues when running ADPrep for Exchange 2010 upgrade to 2013

UPN Issue when running AD Prep as per MS guidelines in readiness for upgrade to Exchange 2013

I get the following error when running  this syntax : setup.exe /PrepareAD ?IAcceptExchangeServerLicenseTerms

Organization Preparation                                  FAILED
     The following error was generated when "$error.Clear();
          $arbMbxname = [Microsoft.Exchange.Management.RecipientTasks.EnableMail
box]::DiscoveryMailboxUniqueName;
          $arbMbxLastName = "MsExchDiscoveryMailbox XXXXXXXX-XXXX-XXXX-XXXX-XXXX
XXXXXXX";
          $arbUser = @(Get-User -Filter {LastName -eq $arbMbxLastName} -IgnoreDe
faultScope -ResultSize 1);

          if ($arbUser.Length -eq 0)
          {
            Install-UserAccount -Name $arbMbxname -LastName $arbMbxLastName;
          }
        " was run: "An Active Directory Constraint Violation error occurred on <FQDN>. Additional information: The operation failed becaus
e UPN value provided for addition/modification is not unique forest-wide.
Active directory response: 000021C8: AtrErr: DSID-03200BF6, #1:
        0: 000021C8: DSID-03200BF6, problem 1005 (CONSTRAINT_ATT_TYPE), data 0,
Att 90290 (userPrincipalName)
".