Vast majority are realizing their gains as rent. A land tax would just be taxing those gains before they can be recycled into the real-estate market. Perfectly just in that case.

If you're talking about an individual not using the land commercially - well theoretically the tax burden lowers the sale price. You don't pay both. At 100% land tax, the sale price would be zero. You pay only as you use the land. That also seems just to me.