My Mac App Store had developed a strange problem: it couldn’t remember that I had installed any of the programs. Thus, it would show the list of all purchases from the store along with the active button that says “Install”, not the button that says “Installed”.
Also, it wouldn’t show updates for any of my installed programs.
I could install any of the programs again — that would work. But almost immediately it forgot that I had installed it. It was like Dory from Finding Nemo.