When I try either of the keys I got from humble, steam tells me that I already own the game, so I assume they are both still valid. Perhaps I copied the wrong key, so I entered the other one now - can you try again?

I now went ahead and verified:

• I never gave away my second key.
• I never activated it myself.
• The key is still recognised by steam.

Unless someone guessed my key, I have no idea what happened...