Hello Timothy:
Long time coming, but you'll be pleased to learn that dhclient (-6) now generates DECLINEs if the client script returns a 3 indicating a DAD error. This change will be included in our next feature release, 4.4.0, due out Q1 of 2018.
We'd like to thank you for suggesting the change by citing you in the release notes. If you wish to recognized in this way, please respond with how you want to be identified.
Regards,
Thomas Markwalder
ISC Software Engineering