Error validating addressing information for "Device-A", IP address clashes with existing device : IP address "xxx.xxx.xxx.xxx" is already used by "Device-B"
This means that both Device-A and Device-B have one of their interfaces configured with the IP xxx.xxx.xxx.xxx.
xxx.xxx.xxx.xxx need not be the polled IP.
A) You can change the IP on the devices' interface.
B) Go to Inventory Administrator>Click "Add.." .
Check mark "Allow duplicate IP Addresses"
and also thanks joby