From the looks of it, I would hazard a guess that this is not the result of a search of the network, but was auto-populated by an unmanaged detector on your network.
If this is the case, then this may explain the multiple entries for the single machine, as everytime an IP address expires and changes on the unknown client, it appears to be getting re-detected by the unmanaged detector as a 'new' unknown client.
The below article says you should be able to see the MAC address of this unknown client, can you confirm if the MAC address is the same on all your duplicates?
http://www.symantec.com/docs/TECH105722