ok, understood.
Both systems not only share sda8 on /var/cache/apt/archives. /var itself is a partition on its own, which I forgott about. I did this to be sure to keep my emails on my lokal mailserver in case I installed a new system.
Now I didn't pay attentention to this fact when I configured my experimental system. When I copied and edited the fstab I forgott about it. Therefore /var/cache/apt/pkgcache.bin was shared, too.
So the problem and the reason for it is understood.
Consequenzes:
1)
My experimental system will become my default system, forcibly, because it is the only system with a valid package database. Thanks to god or the Debian maintainers, the d-u went well. I have to live with the fact that digikam will be uninstallable for a while.
2)
Is there a way to recreate the database for my old default system? It's now ready for experiments (after editing the fstab and unmount /var).
shit happens ...