if you give a winner a key that was already used by mistake, can you give a subsitute game?

No.

You would still get a "Not Received".

You'll either have to give a new key for the same game, or live with having an unreceived in your list.

Giving a substitute game might get the person you plant to gift it to into trouble, as anyone else looking at his/her game list might think that the person has not redeemed the game that was given.

If this happens, so far as I know, you need to make sure they get the game they won from you before the week is out.

if the winner agrees, yes. No one else beside you two will know

No, you can not bribe the winner into marking it as received because you did not have the game you said you did.

You can substitute your duplicate with another copy of that game or take a not received.

You can, but you'll still receive a "Not Received"

I think this 'rule' is completely idiotic. Exactly this happened to me. Won a game (Victoria 2), but the key was used already. The guy who created the giveaway said he received the key from someone else who apparently scammed him, apologized and offered to give me Geneforge Saga (both V2 and GS are bundle games, both cost 20$ on Steam and thus give 20$ cv) instead. It would have been completely childish and idiotic not to accept that offer and instead mark his ga as not received. So I did. After I found out there are a lot of denouncers and wannabe Sherlock Holmes around here I simply got a copy of Victoria 2 from somewhere else to avoid any trouble. But I firmly believe I did the right thing.

I don't see anything wrong with a substitute game as long as it is equal in terms of cv (eg you can't give a bundle game as replacement for a non-bundle game or a game with a cv of 10$ for one with a cv of 50$). Other than that all you have to fear are the denouncers who will scream for a suspension without even talking to you.

Of course you can, but you still have to give the game from the giveaway.

Well the giveaway was meant for said game, so replacing it would be.....difficult, since you're not trading anything, and the winner has the right to claim what he won, granted yould could accept the gift and even mark it as recieved, however some problems could arise from that.

Me personally, i would try to get the game even if i had a "replacement", as the giveaway created was meant for that game, even if that costs me twice the money, or the trouble of contacting support to get it fixed.

This is something interesting that (i think) should be covered in the rules with a bit more detail, since there doesn't seem to be anything written about it.

EDIT: I'm thinking about it some more, it could very well be easily exploitable, so i'm guessing that you HAVE to give the original game away or face the negative feedback.

Heck, if a Fortix key was used and you substituted Skyrim for it, I'd find it within myself to forgive you.

my personal opinion on this
feel free to give a substitute game if the winner wants it, but still go through support to get the giveaway deleted, only give the substitue as an apology not a replacement

Sheee, no one needs to know the key was in reality used already. ;)

I had that happen to me once, he was deeply sorry and gave me another game of greater value...

We later both got a temporary time out by the mods! :D
"It's still against the rules"

Probably not, if I give you this key: ZFJE9-8YL8X-G08WW for Dear Esther, it isn't the same as getting a Little Infero Key (like this one NHHJW-7ZXPR-7DZDY) because they are different games. You have to give them the game that you said they'd win, even if you have to buy a new one.

Happened to me once. Was giving tomb raider away but the key was already taken. So what i did was buy the game and gift it to the guy. Even though it might be "idiotic" or "too serious" Rules ARE rules. They are put for a reason. Without it, a lot of people would exploit and cheat others. It must of happened before. That is why the rule was put into place.

The agreement between the gifter and the winner is their business. If the gifter wants to give you a substitute game, sure you can accept that.

HOWEVER, you need to inform support that you haven't received the game indicated in the giveaway, and was offered a substitute. After that, we can either delete the giveaway (since the game was no longer available) or, depending on the case, put it in a queue for game change. Otherwise, you'll be suspended for not activating won gift, and again, depending on the case, both sides may be suspended for feedback fraud, if you've given a Received mark for a game you didn't get.

Shortly, if you're going to mark a giveaway of a game you didn't get as Received, you're going to have a bad time. In cases like this, contact support and we'll discuss about what can be done.

You can't give him a substitute game, but you can give him a prostitute game. That's how it works.