The allow_duplicate_ip check is in /etc/cobbler settings and you can turn it off.
It will just break DHCP management (manage_dhcp) if enabled in settings, so that's why it's on by default. If you are not using the DHCP auto management feature you should be able to toggle at will.
This may cause some problems with PXE-by-IP (it would be indeterminate which machine would get reinstalled) but as long as the MAC addresses are in there and unique still the PXE specification indicates the MAC will be used first.
I guess I would like my cake and to eat it.
It would be nice to have duplicate detection (ip/hostname/mac) for most entries and to be able to override detection for specific instances.
I was thinking of an --allow-duplicate option - although this might need quite a lot of work?
For now turning it off is ok, but it would be nice if it then issued a duplication warning message.
Happy to have a look at a warning message if you think it's a good idea?
Dave