IPv6 interfaces seem to have hardcoded Link Local Address
The link-local address of each non-WAN interface seems to be hard coded to fe80::1:1. This causes a problem when multiple pfSense routers are connected to the same layer 2.
An example of where this would be a problem would be when you have multiple WAN interfaces with different prefixes being used for redundant paths. You can not have multiple nodes using the same link-local address on one network.
One solution would be to use the same process as is used on the WAN interface Link-Local Address on all of the interfaces. This appears to be via SLAAC and but regardless it should perform Duplicate Address Detection.