How do you set up a redundant server duplicating the primary server and duplicating its services in case of failure or shutdown? I heard it could be done using two VM lic #.
It depends on what you mean with “duplicate” and “services”… this is a very diverse topic. If you mean duplication in certain intervals, you can set up to separate Virtualmin instances and use their Backup & Restore features to get domain data from the primary to the backup server (having the backups stored in a location which both servers have access to).
If you mean realtime duplication, i.e. replication, that’s more difficult and you need to look at each service individually. E.g., web page contents could be replicated using rsync and a script that is triggered whenever files change. For database contents, you can use MySQL master/slave replication. Emails can, since they’re stored as files, be treated like web page contents. And so on and so on. 
I had one server running webpages, database, dns, email etc. with a only a few users. I could shut it down for hours at a time to do admin and stuff and no one would miss it. Now with a moderate number of users if the server is down for a few minutes I get angry emails. I want to add another server so that I can withstand heavier user volumes, continuous use when one machine is offline and automatic sync of the latest changes between the two machines. I saw a post of the procedure some time back in these forums, i think.