You've just finished installing and configuring all those packages on your first machine. Now it's just a matter of cloning the hard drive into all the others and changing the hostname. Right?
Well, not so easy. If you wish to avoid conflicts on your network, backups and elsewhere, i.e. if you wanna end up with a unique fingerprint for each machine, you must go a few steps further.
Here follows a checklist to help you out. It's aimed at Debian and derivates (e.g. Ubuntu), though other distros won't be much different.
The old way: edit /etc/hostname and /etc/hosts