pfSense

In DHCPv6, the client identifier is not the MAC Address anymore. That concept is replaced with the DUID. If the same system has multiple interfaces (ex: an Ethernet one and a WiFi one), both may have the same DUID and will have to be distinguished using their IAID (an Interface ID).

As of now, it is impossible to reserver an IP address in pfSense's DCHPv6 server for each interface in a single system. pfSense only offers to reserver per DUID, without consideration for the IAID.

The need would be to add the option of duplicated DUID and using the IAID to distinguish between these cases.

See screenshots about duplicated DUID with separate IAID here :
https://forum.netgate.com/topic/144154/dhcpv6-static-leases-how-to-set-uniquely-per-interface-duid-iaid

See infos about how Juniper can handle this :
https://www.juniper.net/documentation/us/en/software/junos/subscriber-mgmt-access/topics/topic-map/dhcpv6-duplicate-client-management.html