A good way to loose credits right now is to accidentally buy a map or nano that you have, and try to install it.
How about a comment at the bottom of the descriptions of these things that says "You have this item installed." This way, checking it at the machine, we don't accidentally buy something we already have.
Either that or auto-alpha the ones we have so searching for the things is easier. That'd probably be better.
Once you get more than a few of either, it is rather easy to lose track of what you have, and I'm only a level 52.