Mavericks in Appstore twice?
The OS X upgrades on the MAS are very different than all other products.

It is an installer as opposed to the actual program that you run.

What is available is the chance for you to re-download the installer before you need it and save a copy. Apple initially made it hard for people to re-download the installer - presumably since many are equally confused as you are. However, lots of people were confused on how to re-download the app and a change was made a while back to show it as a download to anyone that "purchased" an upgrade.