Cannot install Server App, Store says "Already installed", but it is false! How to remedy?

Hello,

I had to reinstall Mountain Lion OS on my Mac Mini. I used an external hard disk as the boot disk.

I use this machine as a server, so I tried to install the Server App from the Store.

When I open my Purchases, it says Server App is "Already Installed".

It is not true! There is no Server App installed.

How can I force download and install the Server App?

Please help

thank you Stefano

Also XCode says "Already Installed" but it is not, by the way.