@swinster said in Duplicate DHCP leases:

That device obtained a lease immediately for 172.16.199.17 (as expected) but interestingly in the pfSense GUI, it shows as offline, which it very much isn't.

That Online/Offline status is determined by whether pfSense itself has an ARP entry for that host. It will only have an ARP entry if it has passed unicast traffic with that host from the firewall itself in the recent past. It is perfectly normal for that to show offline. Ping it from the firewall and it will show online if ARP is resolved.

I believe the more-recent linux DHCP clients are doing more with the machine-id than other clients do. I have seen that behavior before as well.