installation – If Cygwin is installed, selecting “Download without installing option” seems to skip installed packages

I tried to build a local installation directory for servers whithout internet connection, from a computer which has already Cygwin installed.

Using this directory leads to an incomplete installation, close to an empty stub (even mintty was missing). I selected “Install” for all packages in the category view during setup, but nothing was installed.

Rebuilding a local directory after having uninstalled Cygwin solved the problem. Did I miss something?