The operation couldn't be performed because '<mailbox display name>' matches multiple entries.

A hybrid remote move fails because a soft-deleted or inactive mailbox in Exchange Online has the same email address.

Why it happens

A soft-deleted mailbox (one that was removed but can still be recovered) or an inactive mailbox (a soft-deleted mailbox kept because of a hold) has the same email address as the mailbox you are moving. The error appears whether you start the move from the EAC, New-MigrationBatch or New-MoveRequest.

How to fix it

1. 1

   Step 1

   In Exchange Online PowerShell, confirm the conflict. Soft-deleted and inactive mailboxes have OU=Soft Deleted Objects in their distinguished name.

2. 2

   Step 2

   Get the GUID of the active mailbox with Get-Mailbox. This command does not return soft-deleted or inactive mailboxes.

3. 3

   Step 3

   Start the move with New-MoveRequest and the GUID instead of the email address. You must use this cmdlet, not a migration batch, for this workaround.

Get-Mailbox -Identity user@contoso.com -SoftDeletedMailbox -IncludeInactiveMailbox | fl Name, Alias, DistinguishedName, GUID, IsSoftDeletedByRemove, *Hold*
Get-Mailbox -Identity user@contoso.com | fl Name, Alias, DistinguishedName, GUID
New-MoveRequest -Identity <active mailbox GUID>

Source

Full guide: How to migrate from Exchange Server to Exchange Online →