I'm not very well versed in legalese, but rabbitvcs-core does include the following files: /usr/share/doc/rabbitvcs-core-0.13.3/AUTHORS
/usr/share/doc/rabbitvcs-core-0.13.3/COPYING /usr/share/doc/rabbitvcs-core-0.13.3/MAINTAINERS
The rest of the subpackages require explicitly installing rabbitvcs-core, so copying COPYING into them is unnecessary.
So, if I understood correctly, it's a false positive for rabbitvcs-core too. (There's no "rabbitvcs" package btw, that might have triggered it?)