2 networkcards in 1 system

[Pieter Donche]

I installed FreeBSD-amd64 on a system which has 2 networkcards and configured
1 networkcard with its IP parameters.

Now I want to add the second networkcard, and did:
# sysinstall
  do Post-Install Configuratio
   Configure additional Netwerk Services
    Configure additional network interface
   and selected em1 and gave it its IP parameters
    (I used the same hostname macos.cmi.ua.ac.be, in the DNS servers here
     macos.cmi.ua.ac.be is assigned to two IP addresses)

(After rebooting,  em1 appears in the boot messages with the correct
Mac- and IP address.. )

But I get many messages:
Jan 13 11:05:25  macos kernel: arp: 143.129.XX.YY is on em0 but got reply
from 00:1b:21:UU:VV:WW on em1
Jan 13 11:54.25  macos kernel: arp: 143.129.XX.YY is on em0 but got reply
from 03:ba:14:UU:VV:WW on em1

( 143.129.XX.YY is our gateway or other system, the MAC address
is the mac-addres of our gateway or that other system )

Why is this? How to remedy?

Or do I need to 'deconfigure' em1? How can I do that?