So you’ve done an apt-get install on a package that was already
automatically installed. What if you don’t want to uninstall it, but
simply remove the “manually installed” mark? The solution is simple:
apt-mark auto zlib1g-dev
20200229/DuckDuckGo dpkg unset manually installed
20200229/https://serverfault.com/questions/87933/how-to-remove-the-installed-manually-flag-and-revert-to-automatically-install