Shut down the affected computer (keep it shutdown); either clear the arp table or reboot the fgt. Wait about 5 mins then perform a ping to the affected IP address then perform a "get system arp" or "get system arp | grep <IP address>". If there is a MAC Address showing up then there is a device configured for a static IP. Otherwise there is likely a problem with the computer's network card/TCP stack, etc. (e.g. try swapping out/in a new NIC.) Other things to check for is a loop (but here should be other signs for that). [strike]If the Fortigate has device monitoring/logging enabled, try disabling that[/strike].
Edit: the above is just some quick and simple suggestions.
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