The value for LegacyDN must be unique to each user.

[TomasHermansson]

Trying to connect user accounts with the mailboxes but I got errors like this:

Connect-Mailbox -Database "Mailbox Database 0989984251" -Identity "Tomas Hermansson"

The LegacyDN "/O=<org>/OU=<org>/CN=RECIPIENTS/CN=<user>" of "b2e288bf-8706-4367-8db8-4686983865f0" is in use by the following user in Active Directory: <username>". The value for LegacyDN must be unique to each user.

    + CategoryInfo          : NotSpecified: (b2e288bf-8706-4367-8db8-4686983865f0:MailboxId) [Connect-Mailbox], MdbAdm

   inTaskException

    + FullyQualifiedErrorId : 613AE561,Microsoft.Exchange.Management.MapiTasks.ConnectMailbox

    + PSComputerName        : <server>

Thanks a lot for any advace!

[#2]

The legacyDN of the mailbox and matches the legacyExchangeDN attribute of the user object in Microsoft Active Directory. Not sure what you did to get into this scenario. Maybe this can help to resolve: https://social.technet.microsoft.com/Forums/exchange/en-US/4474b1d4-28e1-4822-a8fe-4f468266d7f8/disc...

[#3]

If you are trying to connect a disconnected mailbox to a different account with a different legacyExchangeDN than the mailbox originally had, use connect-mailbox with AllowLegacyDNMismatch switch which will bypass this legacyExchangeDN check.