Resolving HA cluster chassis ID conflicts
In a FortiGate-6000 or 7000 FGCP HA configuration, if both devices in the cluster are configured with the same chassis ID, the device with the lowest serial number will be shut down. The other device will continue to operate as a standalone FortiGate in HA mode.
You can resolve the chassis ID conflict by restarting the shut down FortiGate-6000 or 7000 and configuring the FortiGate-6000s or 7000s with different chassis IDs. Once both devices are operating in HA mode with different chassis IDs, the devices will negotiate to form a cluster, and if their chassis IDs are different the cluster will begin to operate normally.