I’m working on some outdated memories, but IIRC:
Germany allows dual citizenship now, but used to not allow it in most cases. In those cases, if you applied for German citizenship, you had to express that you were willing to give up your old citizenship. Once you were granted citizenship, you had a certain amount of time (two years?) to show a certificate that you renounced your old citizenship. If you didn’t, your German citizenship would be revoked.
Last I checked, it was out of print, so I bought Unfathomable instead (reimplementation in the Lovecraft universe). Still good, but missing pilots, so not as good :).