The original source code from ISC does not support such requirement for IPv6. But, it was fixed in Fedora in 2010. See http://www.mirrorservice.org/sites/dl.fedoraproject.org/pub/fedora/linux/releases/20/Fedora/source/SRPMS/d/
But this patch has a bug. It uses ALL options from REQUEST to send DECLINE, however RFC3315 says no Rapid_Commit and no Reconf options. Be careful.
Main idea is to check leased IP for duplication in dhclient-script. If the IP is duplicated script should return 3 otherwise 0.