In practice installation script of an external package does --del-package before --add-package, because if it was installed before, --add-package would cause an error.

Wouldn't be reasonable then to let --add-package overwrite any existing package of that name? There is no real safety, because the package is explicitly deleted anyway.

I don't think I agree. Installation of an external package shouldn't be doing --delete-package first: that should be left to the user (or rpm -e, or whatever). If you want to install a new package that replaces an existing one, you have to remove the existing one first. Cheers, Simon