Fortimanager Error: A device with Serial Number already exists

Description
When trying to add a device to FortiManager, the following error is displayed "A device with Serial Number already exists"

Scope
Fortimanager


Solution
There is another device (either registered  or unregistered) with same serial number in the FortiManager's device list. 

Note: An unregistered entry will occur if the FortiGate has already attempted to connect to FortiManager.

# diagnose dvm device list

Identify registry with serial number and delete it

# diagnose dvm device delete "adom"  "device name"

After deleting the existing device, you be able to add the new device to FortiManager.