Not as full featured or elegant as Jack's- SQL
Select Computer.DEVICENAME, count(*) from computer group by DEVICENAME having count (*) <> 1
I really all depends on how your duplicates are showing up. If you have clients getting new device ids (such as reinstalling the client after deleting the appropriate data), then you will see the same device name showing up more than once. If you are reimaging machines, then the device name is likely going to be different and so you need to use different attributes, such as the MAC address (although there can be valid duplicates, such as VPNed machines) or serial number (as long as the serial number actually gets put in the bios).
To really find the duplicates you need to do more than just look at device names, although in most instances that will get a high percentage of duplicates, depending on how the duplicates are getting in.
Thanks guys, Most of our duplicate devices are being created when replacing an existing device that has failed. When a POS workstation fails in the field it is replaced using the same name. We do have proceedures for removing devices from the DB before provisioning a new system but as we all know, people don't always follow the proceedures as prescribed. Usually I find quite a few of the duplicates by sorting on the last hardware scan date. Most of the time, duplicates (old systems) show up having a old scan date as they no longer update the DB. I will then sort by the device name and sure enough, I will have two of them which at that point I remove the old one.