dhcpd 3.0 lease unavailable error

Our dhcpd server, running ISC's dhcpd 3.0, seems got lots of following
messages recently,

DHCPREQUEST for xx.xx.xx.xx from 00:00:00:00:00:00 via eth0: lease
xx.xx.xx.xx unavailable.

where xx.xx.xx.xx is the assigned ip address for that mac, according
to dhcpd.leases. It happen randomly to all users, and this messages
seems to be increasing. I've check the dhcpd.conf and dhcpd.leases
files, everything seems to be in order.

Any idea why this is happening? and what's the outcome from it (we
have users reporting dropping out from the leases)?

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