If I insert valid cd-key in steam and I already own the game will the cd-key will become duplicated?

I found a cd-key without a name to it in my "CD-KEY" text file on my computer. And I said to myself I better activate it and after i did the this message apeared "Product Already Owned". Apparently the cd-key is for "To The Moon". Here is the funny part i won "To The Moon" from SteamGifts 1 week ago and now I found that have a cd-key for it for such a long time.