OK, the plot thins slightly. It appears that when you run the inventory scan from the client it updates the MAC address as it should and removes the duplicate. I unfortunately cannot run a Full Sync Scan from the core on these devices as it cannot see them?
It seems that as these are tablets and sometimes not in use / asleep that a manual scan from the core wont work.
I will have to find the users of these devices and run an inventory scan, hopefully this will wipe out the MAC address leaving the adaptor for builds again.
Thanks for the help throughout, this is a new process within our environment and it looks like a manual scan from the device without the USB adaptor in after it has been built will fix it afterall.