List of installed packages to backup /recreate installation

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http://askubuntu.com – Sorry i found how to post in Ubuntu Forum :) , simple "new Thread" button, LOL !!!. Anyway i can leave this question open here i think. The following command create a list of installed packages on a machine, and then reinstall the same packages in another machine it seems that the command work fine on Ubunt 32 Desktop , but on Ubuntu amd64 (Desktop) the sudo apt-get dselect-upgrade tell me that he need to uninstall a lot of packages (even unity unity-2d unity-common!! ) even if in the list of packages appear those packages. Even if those command are give in sequence on the same computer (HowTos)