pfSense

Tested on:
23.09.1-RELEASE (amd64)
built on Wed Dec 20 18:27:00 UTC 2023
FreeBSD 14.0-CURRENT

I can confirm this behavior

I can confirm this issue. I also can confirm that it happens with both the Allowed MACs and Denied MACs fields. You can have your MAC address in the same field on two or more interfaces or one in each opposing field on each interfaces to trigger the issue.

Tested against:

24.03-RELEASE (amd64)
built on Wed Apr 24 17:38:00 UTC 2024
FreeBSD 15.0-CURRENT

The issue persisted.

May 4 07:19:22    kea-dhcp4    265    ERROR [kea-dhcp4.dhcp4.0x23dc89412000] DHCP4_INIT_FAIL failed to initialize Kea server: configuration error using file '/usr/local/etc/kea/kea-dhcp4.conf': Can't add class: Client Class: mac_1C57DC603576 has already been defined (/usr/local/etc/kea/kea-dhcp4.conf:62:13)