I never thought this day would come, but with all the great indie bundles, gog.com, Steam, Desura and whatnot I managed to amass too many games to actually remember whether I already bought a certain one on another site when I see a great offer / bundle... So yes, this is the real disadvantage of the digital age, no more boxes which you remember already having in your shelve (also, you'd probably throw some away every now and then).
Steam has that nice property of showing you which games you already own on Steam so you can't accidentally buy the twice (except as gifts), but for some games I own another license. So, how to keep an overview? How to avoid wasting money for an apparently great offer which actually wouldn't buy something new?
Is there a one-for-all game management program?