Playing around with the idea of building a query for duplicate Mac address and so far "duplicate" is only offered on System Name. So perhaps a workaround would be to build a query that displays the Mac address and export out as a csv file. At that point you can filter for dup's...Just a thought..
Another idea is build this report:
This will produce a report of MAC address by count. The majority should all be 1, and thus, not important to your troubleshooting. Anything 2 or greater you can click on the MAC and get a list of the computers, which you cna then export.