We have the following setup:
http://planet.symlink.ch/ is subscribed to http://noone.org/blog/index.rss
http://planet-commandline.org/ is subscribed to http://noone.org/blog/English/Computer/Debian/CoolTools/index.rss and http://noone.org/blog/English/Computer/Shell/index.rss which both contain subsets of the postings in http://noone.org/blog/index.rss
Both planets run from the same PlanetVenus installation (git master HEAD as of now) including the same cache, just with different configuration files.
Now the posting http://noone.org/blog/Deutsch/Computer/Internet/Mutationen%2520auf%2520Planet%2520Symlink.html shows up on http://planet-commandline.org/ as soon as it has been updated once after http://planet.symlink.ch/ has cached it, despite it's only in http://noone.org/blog/index.rss but neither in http://noone.org/blog/English/Computer/Debian/CoolTools/index.rss nor in http://noone.org/blog/English/Computer/Shell/index.rss
Looks to me as if the cache evaluation is broken with regards to checking if a cached posting belongs to a planet or not.
A workaround is to use a non-shared cache directory for those two planets, but this also means that at least one feed is fetched twice now.