Edit: sorry, I misread.
Does it mean that the DHCPv6 Clients on the different pppoe-out interfaces all have the same DUIDs, but different from when use-interface-duid is unchecked, right?
Edit 2: Well, while trying to create additional PPPoE out instances and adding DHCPv6 clients to them, I have now been hit by this bug 
:
http://forum.mikrotik.com/t/v7-16-ipv6-and-dhcp-client/179127/1
http://forum.mikrotik.com/t/disappearing-ipv6-configuration/179434/1
Looks like I have to reset everything now.