man dhcpd.conf в частности говорит:
"The duplicates keyword
allow duplicates; deny duplicates;
Host declarations can match client messages based on the DHCP Client Identifier option or based on the client's network hardware type and MAC address. If the MAC address is used, the host declaration will match any client with that MAC address - even clients with different client identifiers. This doesn't normally happen, but is possible when one computer has more than one operating system installed on it - for example, Microsoft Windows and NetBSD or Linux.
The duplicates flag tells the DHCP server that if a request is received from a client that matches the MAC address of a host declaration, any other leases matching that MAC address should be discarded by the server, even if the UID is not the same. This is a violation of the DHCP protocol, but can prevent clients whose client identifiers change regularly from holding many leases at the same time. By default, duplicates are allowed."