PS C:\> Get-MigrationUser firstname.lastname@example.org | FL Identity : email@example.com BatchId : Pilot Migration 1 MailboxEmailAddress : firstname.lastname@example.org RecipientType : MailboxOrMailuser Status : Failed StatusSummary : Failed MigrationType : ExchangeRemoteMove State : Failed WorkflowStep : DataMigration WorkflowStage : Injection ErrorSummary : The onboarding move could not be created because user 'User' is already being moved.
If you have already checked all your migration batches (Get-MigrationBatch) and/or all your individual move requests (Get-MoveRequest “user”) and there is absolutely no move request associated with this user, then it is very likely you still have a move request for the user in your on-premises Exchange environment.
So, go to your on-premises environment and remove the local move request for that particular user. This should clear the msExchMailboxMoveRemoteHostName AD attribute. Other attributes related to move requests that you can also check are: