Connect-Mailbox and AllowLagacyDNMismatch

I have solved a one case where we wanted to connect (Use the Connect-Mailbox cmdlet to connect a disconnected mailbox to an Active Directory user object.) an archive mailbox to a linked mailbox in Exchange 2013.

Error from EMS:

25- 7- 2014 10-32-07

[PS] C:\>Connect-Mailbox -Identity "Personal Archive - Jan Novak" -Archive -User JanNovak2 -Database "EXDAG1-DB03"
WARNING: An unexpected error has occurred and a Watson dump is being generated: Object reference not set to an instance
 of an object.
Object reference not set to an instance of an object.
    + CategoryInfo          : NotSpecified: (:) [Connect-Mailbox], NullReferenceException
    + FullyQualifiedErrorId : System.NullReferenceException,Microsoft.Exchange.Management.MapiTasks.ConnectMailbox
    + PSComputerName        :

Error from GUI:

The LegacyDN "/o=CONTOSO/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=Jan Novak" of "cb67270a-ada2-472e-94cc-eb7140f3520f" is in use by the following user in Active Directory: "Pepa Novak (new)". The value for LegacyDN must be unique to each user.

Obviously, the LegacyDN attribute was not unique and defended to connect the mailbox.

[PS] C:\> # disconnected archive
[PS] C:\>(Get-mailboxdatabase|Get-Mailboxstatistics|?{$_.DisconnectReason -ne $null -and $_.DisplayName -like "*Jan*"}).LegacyDN
/o=CONTOSO /ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=Jan Novak

[PS] C:\># linked mailbox
[PS] C:\>(Get-mailbox JanNovak2).LegacyDN
/o=contoso/ou=exchange administrative group (fydibohf23spdlt)/cn=recipients/cn=Jan Novak

How to connect mailbox in case this case? According to Technet and Connect-Mailbox (Exchange 2013) there is available a switch:

25- 7- 2014 10-23-48Well, we have available the switch for similar scenario (i.e. AllowLagacyDNMismatch), but it is reserved only for MS use. I do not know why is it so (let me know if you a clue), but it worked for me without problem:

[PS] C:\> Connect-Mailbox -Identity "Personal Archive - Jan Novak" -Archive -User JanNovak2 -Database "EXDAG1-DB03" -AllowLegacyDNMismatch

[PS] C:\> Test-ArchiveConnectivity

RunspaceId               : 7df20326-4fc0-4ca3-877f-5273aea0d5b7
Identity                 :
PrimaryMRMConfiguration  :
PrimaryLastProcessedTime :
ArchiveDomain            :
ArchiveDatabase          : EXDAG1-DB03
ArchiveMRMConfiguration  :
ArchiveLastProcessedTime :
ComplianceConfiguration  : ElcV2
ItemMRMProperties        :
Result                   : Successfully logged on to the users Archive mailbox.
Error                    :
IsValid                  : True
ObjectState              : New