There is no specific agent scanning setting that would tell the scan to only look at BIOS serial number, its not the agent that creates the device/duplicates, that is what Snow Inventory is doing when it receives and processes the snowpack manifest. It will attempt to process the data using 4 unique properties....site name, hostname, BIOS Manufacturer and BIOS Serial Number....

If a change is detected in one of these, assuming in your case, an already existing device getting its hostname change, this will trigger a duplicate (it says 2 properties have to change but I'm not so sure, pretty sure it occurs with just one). There is an option to configure Snow Inventory to only look at the Hostname (and site name), this called HostnameOnly mode. Once configured, providing your machine name doesn't change once provisioned, it shouldn't create duplication. By the sounds of it, your process of re-imaging your device and creating a new hostname will cause duplication regardless of either option. In that case you may have to manage the duplication by implementing some sort of de-dupe process, an option which is described here.