Watch the network for an IP conflict - ipwatchd

Installation on Ubuntu:

sudo aptitude install ipwatchd-gnotify

if configured correctly (true by default) it will show you graphical warnings for IP conflicts as well as create log entries in /var/log/syslog (check for log messages containing ipwatchd).

Advanced configuration options are described and have to be set in the file/etc/ipwatchd.conf. This could be running a custom script as soon as an IP conflict is being detected for example.

When you detect an IP conflict, it might be an illegitimate attempt to take over your IP. So you want to tell the network who's the real owner of that IP: this is done using arping and described in my blog post React to IP Takeover.