If I import workstation information should it update existing records?

I ask as I have a batch of workstations that need their names and domain changing and at present we can't scan them to get the update automatically. I had expected that if I prepared a CSV import file with the new workstation name and old Service Tag in it the import would note that it already had a device with the same service tag and update it. What I found was 2 assets in the system with the same service tag, the old and new device.

Regards,

Andy