@JKnott SLAAC isn't a problem since only the prefix gets provided by the RAs and then the system generates its own address in such away as to ensure no conflicts. DHCP6 also isn't a problem if you just need dynamic address allocation, that also work fine for different interfaces since the DHCP6 server can use the IAID values to distinguish different interfaces on the same host.
The issue is if you use DHCP6 together with static leases (think DHCP4 reservations where an IP address is tied to a specific MAC address). A DHCP6 client uses the same DUID for all DHCP6 requests (since the DUID identifies the host) but each interface uses a different IAID so the combination of DUID+IAID uniquely identifies a specific interface on a specific host.
Unfortunately, in pfSense when you create a static DHCP6 lease to tie a specific dynamic IP address to a specific interface pfSense only allows you to define the DUID not the IAID. So if I have a macOS system, say, with a DUID of, say, 00:01:00:01:2b:b9:95:35:5c:e9:1e:e4:6b:67 and I define a DHCP6 static lease for that DUID with an address of, say, fd00::25/64 then if the macOS system connects to the network via Ethernet if gets assigned that IP address. If its WiFi interface also then connects to the same network it also gets assigned the same IP address -> very bad.
If pfSense allowed you to define both the DUID and IAID as part of a static lease then this issue would be resolved. Sadly it does not, which seems like quite an oversight...