It's a tax on land itself not including improvements that's designed to tax landowners based on the value that being located where it is in the community and in relation to shared resources adds to the property. The idea is that this is relatively fair compared to other tax schemes in that it (ideally) maps closely to the value the landowners are deriving from the taxes they're paying, and encourages high levels of development on land with good infrastructure and in good locations. It's also easier to collect than many other forms of taxation, which is nice.
As covered in the Wikipedia article, the main problem with it is accurately assessing the value of the land. Which is kind of a big deal, obviously.
So it would be a double tax since the current property tax already accounts for value of location. The appraised value used is not just the improvements but the overall value which would include things like location.
How does taxing one fewer aspects of something make it a "double tax"? It's not a "half-tax" either, but at least that would make more sense, unless I'm missing something.
