As time goes new kernel versions are released. After a kernel update old installed versions are not removed automatically, they are keep in case something goes wrong so you can boot again to a previous version.
To install a package signing key you have to use the apt-key add command. For example if you want to add the Google signing key:
apt-key add
wget -qO- https://dl-ssl.google.com/linux/linux_signing_key.pub | sudo…