If you already own the game, activating it again won't do anything. The key is still valid, but you do not yet know what game it is for. In the product activation tab for Steam (in your browser, not the Steam client) check the confirmation box, press F12, then input your code and click activate. You will get that same message that you already own the product. On your right, or otherwise at the bottom of your screen there will be a few different tabs. Click on the network tab. When you click activate, a new entry will appear. Click on that entry and click the down arrow. You should then be able to find the game TITLE somewhere in that entry.
If you already own the game, then you can probably use the key for a giveaway on Indiegala, give it to a friend, or if the game is good, you could sell or trade it.
If, however, you get a different error, where it states that the key has been activated by another account, then the key is not valid and you got unlucky.