When you move mailboxes using migration batches you might encounter a situation that your batch contains migration users that fail during batch execution. One of the possible reasons is an existing move request for the affected users. You must remove those requests to successfully move mailboxes.
The following PowerShell example gets all failed migration users from a migration batch and removes existing move requests.
$r = Get-MigrationUser -BatchId MyMigrationBatch | ?{$_.status -eq 'Failed'}$r | %{Remove-MoveRequest -Identity $_.MailboxIdentifier -Confirm:$false}
Enjoy Exchange Server!
