I started up KSP today and began throwing together a manned sample return mission to Minimus. Then I realized that I might have already done this, but I wasn't sure. It would be nice if there was a place where all of your previously obtained data was logged. Maybe have another building called the Data Center. When you click on the building you're presented with a screen. The screen has a tab for each scientific instrument you've used, crew reports, samples returned, EVA reports, etc. Clicking on a tab will present you with a list of all measurements taken by that instrument (or crew report or whatever). This way if you want to know if you've already obtained samples from Minimus you can click on the Data Center, click on the Sample Returns tab, and look for an entry for Minimus. If it's there then you know you've already done a sample return mission. If not, then you haven't. It's pretty easy to implement too. Just send a message to the Data Center saying to add the data each time you successfully transmit data or recover a vehicle.
