DHCP Conflict Detection Attempts

Ok so I know what it's used for, but was wondering what if I had clients from another company with the same IP address block, lets say 172.16.0.x in both offices come together. All clients are running Windows XP Pro.
Question is, will conflict detection be able to bypass the firewall? I know by default XP Pro blocks ICMP echo requests? Is this true?
If it is, is there a group policy or something to enable ICMP?
Question is, will conflict detection be able to bypass the firewall? I know by default XP Pro blocks ICMP echo requests? Is this true?
If it is, is there a group policy or something to enable ICMP?
Comments
Conflict detection uses regular 'ol pings, so they will probably be blocked by default.
You should use both. How does only using a hardware solution for the internet protect clients from internal threats, such as other employees or someone plugging into a jack or hopping on the WiFi? Software firewalls have numerous benefits.
You can centrally manage software firewalls as well.