[SOLVED] The product has already been activated by a different Steam account

I would ask for a screenshot from Steam showing the steam key was already redeemed message. This will be proof that the key was already used.

I doubt Humble gave you used keys especially if nobody else has access to your account. There might be a mistake by the winner such as pasting wrong key or including an extra space.

If it is from a humble choice you should check the corresponding sg thread if others reported about already used keys. Also you can try to ask Humble support for a replacement.

You might also be able to contact steam to ask them when the specific key was activated, it's not guaranteed they will but at least worth a try. That way you can contact Humble is the key has been activated before the sell date or open a request received feedback ticket if the key has been actived shortly after you send the key to the winner.

Also note they will definitely not tell you on which account it has been activated.

Ask support, when you did nothing wrong and it was a "new" recently acquired key.. maybe the winner has a history of "already-activated" keys and the other giveaways got deleted... support can help you maybe... but don*t expect a fast answer :-p

You can contact Steam support, and if you are convincing, they will tell you the date and time of activation of the specified key (do not try to find out the account on which it was activated, you will still be refused. I advise you to say that you were sold the wrong key and this information you need to support the store)
In addition, I would ask the winner to write that he agrees to remove the giveaway and just delete it. This would be a great solution (if you don't care about the contribution).