How to locate the culprit of IP Conflict

HI,
We are currently experiencing an IP conflict on our network. Some other device is using the same IP as our firewall. The firewall is still functioning but when we shutdown the firewall to upgrade it and bring it back up we fear it will not beable to bind the address since the other device is using it. We have two pieces of information to work with, we know the IP address and the MAC address of the interface that is not supposed to be using that address. How can we gather more information such as a computer name which doesn’t show up when the IP conflict is reported. Is there a way to track this device down, if so please share your knowledge / experiences. Thanks in advance..!