[CentOS] ISC DHCP failover

Anybody have any experience with setting up dhcpd in failover mode
between two servers?  I set this up on a couple of servers, and it seems
to be working, but I don't think it is working "right".  It appears both
servers are replying to all requests (which for renewals works okay
because they both give the same address, but new requests get two
different responses).  I thought that only one server would reply to a
particular request.

Also, every DHCPACK is followed by a message like this in the log:

Sep 23 15:45:50 rad2 dhcpd: bind update on x.x.x.x from mypeer rejected: incoming update is less critical than outgoing update

Any ideas?  I subscribed and asked over on the ISC-operated dhcp-users
list but haven't had any responses.  Google finds others asking about
the same log message, and the only responses seem to be "well, if you
get it for every update, there's probably some configuration issue" (but
nobody ever says what issue might lead to it).

-- 
Chris Adams <linux at cmadams.net>