This "the money was already taxed" line is so much BS. There's not some law handed from God that says "Thy shall only levy taxes on a single dollar once in its entire existence". There isn't even one that says "Thou shalt not levy a tax on this meagre dollar more than once as long as it is held by this particular individual".

Taxes are a matter of choice - a (democratic, even vaguely) society makes a choice to spend a certain amount of money, and as a reflection of that, they levy taxes on a variety of things to raise something roughly equal to that amount of money. These taxes will affect different people differently, and they will affect different kinds of income and different kinds of wealth differently. Some taxes are based on what a person is paid for their labor. Some taxes are based on a person's ownership of something valuable. Some taxes are based on what a person spends. What matters is who pays what and why, not whether "the money was already taxed".

But you're right that we have a choice in whether or not we levy a tax on the ownership of land/buildings. Personally, I'm more a Georgeist in such things, and I think that taxing the ownership of land is an excellent idea, much better than taxing income or spending. But I don't feel that Georgism itself answers the question of how to determine how much tax a person would/should pay for owning a particular piece of land.