One Building per building type

I would like to know how to make that a player can only build 1 building of a certain type. I know a couple of ways using some complicated random shit but im pretty sure theres a simple way.

When they build that building check if they already have one on the map
If they do already have it, remove it, refund the money it costs to make, and put up a msg that says something like "you can only build 1 of these"

That would be the easiest way to do it

Thanks