DHCPv6 client report fresh address in use and server ignore 'ignore declines'

[Hillary Nelson]

We moved some of our networks from SLAAC to DHCPv6 today, and there are
tons of log with client complaining that addresses being used while they
are not.
Running dhcp 4.3.2.

For example, I've added a new pool with 2001:beef:2:1018:f000::55a is new
address, few mins later, client reports that address is already in use, and
server marks the address as abandoned even with 'ignore declines'
configured.

Jul 30 19:31:25 serverA dhcpd: Picking pool address
2001:beef:2:1018:f000::55a
Jul 30 19:31:25 serverA dhcpd: Advertise NA: address
2001:beef:2:1018:f000::55a to client with duid
00:01:00:01:18:eb:70:4f:18:03:73:33:6f:af iaid = 277917980 valid for 3600
seconds
Jul 30 19:33:35 serverA dhcpd: Picking pool address
2001:beef:2:1018:f000::55a
Jul 30 19:33:35 serverA dhcpd: Advertise NA: address
2001:beef:2:1018:f000::55a to client with duid
00:01:00:01:18:eb:70:4f:18:03:73:33:6f:af iaid = 277917980 valid for 3600
seconds

Then lots of messages like this:
Jul 30 19:36:41 serverA dhcpd: Client
00:01:00:01:18:eb:70:4f:18:03:73:33:6f:af reports address
2001:beef:2:1018:f000::55a is already in use by another host!
..
Jul 30 19:50:15 serverA dhcpd: Client
00:01:00:01:18:eb:70:4f:18:03:73:33:6f:af reports address
2001:beef:2:1018:f000::d3d is already in use by another host!
Jul 30 19:51:12 serverA dhcpd: Client
00:01:00:01:18:eb:70:4f:18:03:73:33:6f:af reports address
2001:beef:2:1018:f000::e43 is already in use by another host!

I wonder what caused client to report a fresh address in use, the same
problem appear in version 4.3.1. Any help will be greatly appreciated!

Hillary
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