determining duplicate ip address problems

Anyone had this happen? I recently had a remote network where the firewall' s internal ip address what duplicated on a Dlink router which was install on the network without consent of the managing entity! The dlink had the same ip address. I identified the problem by using divide and conquer methods to isolate it down to another device. I cant find any debug commands for arp? Are there any FortOS commands for debugging arp/ip address conflicts that would help me in the future? Any ideas are appreciated! Thanks, Pat