Steam keys used for a product you already had: are they used or free?

If you use a key and already own the game, the key will not be used since you can't own the game 2 times in 1 Steam account.
Just be careful about keys that contain multiple things or base game + DLC since then I think it will use the key to add whatever you are missing.

I think the keys should be still not used, but I am not sure.

I also seem to recall that there have been cases of Steam saying "already owned" falsely, still activating the key. You might want to check your product and licences activation page on Steam to see if the game was added the time your tried the keys.

Maybe also worth mentioning is that steam will give you the "already owned" message even if the key was already used by someone else.

As Darkdreams1 said, if you get the "product already owned" message, it simply means you own what the key would activate, regardless of if the key is used or not. If it wasn't previously used and you give it to someone who doesn't have the game, then it should still work.

A good use case for this is lets say you buy one of those mystery keys from fanatical, if you try to activate the key on your account but then it says you already own it, you now know what the key is and you can safely make a giveaway for it since fanatical is reputable and there should be no issues with the keys you get from them.

If your getting a key from a shady site that tends to give duplicate or used keys, it won't help you with that though.

If you have any doubts about validity of a key and the game is too valuable to just give for free, there is a Lootboy SG group here. It has been founded, because some keys received from LootBoy do not work, but folks often post GAs there with keys from other sources (like IndieGala loot keys), that they do not fully trust.

If you win a giveaway in this group and the key is not working, you have to agree to delete the giveaway. Users who don't agree will be removed from the group. NOTE: Of course this doesn't affect the SG-rules and users could still not agree to delete the giveaway. But like stated above in that case I will remove them from this group so that they can't participate in any more group giveaways.

Thanks for the info. I was unsure and so I have previously thrown out good keys fearing that Steam simply ate the duplicates. :(