By default synaptic copy downloaded file to /var/cache/apt/archives and install.
if it not to install for some reason, you can select the package and again install without the necessity of new download. or he can... open one xterm
apt-get -- reinstall install package
rpm -Uvh package.rpm
Sorry!!! I´m use synaptic and apt-rpm natively. I´m using Conectiva Linux: -)
sorry my poor english :P