The easiest thing to do is search the game, in general, through the main database. If you've added the game to your collection, it will say 'in collection' under the game's title in search results. Or, if you go to the item's page, there's a bit at the top for 'your stuff'. It'll be 0 if it's not in your collection. If it is, you'll see a number for how many are in your collection- so it's a good way to look for accidental duplicates.