When you want to restore your packages that you’ve backup-ed with :

sudo dpkg --get_selections > selections.txt

And when you restore them with:

dpkg --set-selections < selections.txt

And the result is a whole bunch of warnings like  “warning: package blabla not in database….”

Then theres a easy fix:

$ sudo apt-get install dselect
$ sudo dselect 
   -> Update
   -> Install

Done.