Considering the answers I got, I feel my question was not adequat.
I want to clone the same list of installed software, but the base installation may differ
slightly because the two computers do not have the same hardware: different disk
capacity and video card. They also will have different IP addresses..
What I would like is an editable installation script, which I can tailor somewhat (for
example by removing the X Window installation that is likely to be different anyway).
Modifying an existing installation list is a lot less complicated than reselecting all
the packages manually, of which which 90% is pure application stuff.
The solution I am looking for would be to perform the base installation manually
(including X) and go on with the remaining packages (not in the base) in an
automatic (or semi-automatic) way.