Chris,
What does the physical interconnection of cables look like?
Where is the arpwatch daemon?
Do you have your public network and private network connected to
the same hub/switch?
Could you give us a bit of ASCII art to show the configuration?
Judging from the symptoms, the only logical explanation I can imagine is
as follows. You have your public and private networks connected to the
same medium (hub/switch). Now, when a host on this network makes an ARP
request for 198.31.174.56, it might get two answers.
