dovecot - Jan 2015 - Uniqueness of dovecot mailbox-guids

Dovecot version: 2.2.9

I'm using doveadm backup to migrate about hundred user mailboxes from cyrus
to dovecot. I defined a master user for the old cyrus mailserver in imapc
backend to get access to all the mailboxes. Now I realized that after migration
every user's "Inbox" or "Trash" on the new dovecot
server has the same mailbox-guid, e.g:

User a:
inbox dd9456...
trash 5549cc...

User b:
inbox dd9456...
trash 5549cc...

Is this a wanted behaviour? If i do not migrate with doveadm backup and just
manually create mailboxes, the mailbox-guids are globally unique, e.g:

User a:
inbox dd9456...
trash 5549cc...

User b:
inbox c543fe...
trash t49ccc...

Thanks!
Christian