Scenario 2: I claimed a game and found out I already have it or don't want it. / I posted a game and someone claimed it and found out they didn't want it or already have it.
First, please don't enter for games you don't want. If you think it's likely you'll buy the game, just don't enter. Don't enter every giveaway just because it's there. Enter the giveaways you want most. Be fair to other people.
Second, now that the mistake has been made, you can fix it. Send a PM to ModBot with the subject line nuke. In the body, place the keys or URLs you wish to nuke one per line. The keys you should use are the actual, original serial keys, NOT the ModBot MB- codes. You will not receive a response. After a few minutes, you can submit a new giveaway with the key or URL and it will work fine.