I believe I might have found a bug whilst I was laying out a taxiway foundation. If I click on an already built taxiway foundation, the cost to build it is deducted from my balance, but there is no taxiway foundation built.
It shouldn’t allow me to build a new taxiway foundation on an existing foundation, the same way it doesn’t let me build another wall on an existing wall.
Yes, I’ve experienced that too with everything, even decorative floor. It will charge the numbers of tiles you select, no matter if it already is there or not.