How to identify and rectify IP address conflicts in Ubuntu?

You can find them with arp-scan:

sudo apt-get install arp-scan

sudo arp-scan -I eth0 -l will show IP addresses, MAC address and manufacturer of the NIC and the line in the output that is a duplicate also identifies itself with a (DUP: 2) (where 2 is the second time this IP address is found for eth0.

Some possible options to make the search more specific:

• Specify a list of IP addresses as arguments: sudo arp-scan -I eth0 192.168.1.1 192.168.1.2 192.168.1.3
• Specify network/bits: sudo arp-scan -I eth0 192.168.1.0/24
• Specify network:netmask: sudo arp-scan -I eth0 192.168.1.0:255.255.255.0
• Specify address range: sudo arp-scan -I eth0 192.168.1.3-192.168.1.27
• Read a list of IP addresses from a file: sudo arp-scan -I eth0 --file=ip-address-list.txt
• sudo arp-fingerprint -o "--interface=eth0 --numeric" 192.168.1.111 displays the IP address, the binary fingerprint string, and a list of known systems that match this fingerprint: 192.168.1.111 01000100000 Linux 2.2, 2.4, 2.6

You can add |grep {part.of.ip.address} to limit the output (do not use wildcards but regular expressions if you need more exotic combinations).

In addition to arp-scan, the daemon ipwatchd may be useful in some circumstances and is worth mentioning on this question for others coming here via search.

IPwatchD is a simple daemon that analyses all incoming ARP packets in order to detect IP conflicts.

It can be configured to run a user script when conflicts are detected too.

arp-scan is able to detect conflict across an entire network. ipwatchd detects when the hosts's IP address is use elsewhere as well.

As an aside, ARP is useful for this task as uses broadcast traffic which is sent to all hosts, whereas much other traffic is routed only to intended recipients (at the MAC level.)