First off, thanks for the welcome in the other topic! 
This I was wondering myself. 
The database resides in HOME/.config/Duplicati and since I am backing up HOME, it slid into the backup unnoticed. Up to now I have never had problems and even did not know that there was a database and where it was stored. Possibly quite ignorant from my side… 
So I will now move the database outside of the backed-up directories. Thanks for the hint!