So this is the reason for this issue, in that time I have enable the device detection in the firewall.that is the reason for this mess, if you guys want to enable device detection please make sure your end devices are virus-free. because as my knowledge FortiGate detects devices using by TTL. in my scenario my client PC's are exposed to viruses and they change the TTL of the machines,
diagnose user device get <MAC-ADDRESS> (you can get device information device OS etc)
diagnose user device clear all (clear all the device inventory)
diagnose user device del <MAC-ADDRESS> (you can delete one by one)
using the above command you can resolve the issue but it's not permanent.
I solve this quite some time ago sorry for the late reply.