Hello, I've just exerienced the same during an upgrade of sid packages.
The only way to break out of this mess is to force the upgrade of the package with: dpkg --force-overwrite -i /var/cache/apt/archives/libxtables12_1.6.0+snapshot20161117-2_amd64.deb then it becomes possible to continue with the rest of the upgrade. libxtables12 seems like it should have a "breaks: libxtables11", or replaces: (I'm not sure if the latter is actually appropriate in this situation)
signature.asc
Description: OpenPGP digital signature
