Depending upon how many machines you have, you can see the MAC address of the duplicate name it tells you in event viewer, I had this on my network to find that two colleagues were running the same virtual server with the same name and I tracked it by MAC address, I only have 11 PCs so it was not a big task.
One way would be to switch everything off and start one by one until you get the error. It is possible to have a DNS entry for one machine and another with the local name. I did this to get round mapped drive issues etc, called the new server a new name and gave it a DNS alias of the old server once swithced off, if I had put the old server back on again I would have had duplicate names. CHeck DNS also