From the IBM NIM documentation:
Each network communications adapter has an associated network hardware address that is unique. In this case, you attempted to define a NIM network interface using a network hardware address already being used by a NIM machine object.
Only one NIM interface attribute may be defined for each network communications adapter a client might have. If you are attempting to add another interface definition, then verify that the hardware address is correct. If so, then you must first change the interface attribute that is currently using that address. If not, try the operation again with the correct hardware address.
What specifically must we do in NIM to allow us to provision to this device again?
Per Jae Yi: the machine (container) cannot already exist in NIM. BSA won't provision on top of an existing machine, so the machine must be removed first.
nim -o remove <machine name>
Once it was removed, BSA was able to successfully provision to the Device that was previously in use by this machine.