With the fourth Humble Indie Bundle I received second copy of the third pack that I already own. Is there any way to add the key or the games to my account as something gift/trade-able? Duplicate games don't seem to be rendered giftable (I already owned e.g. Super Meat Boy) so I don't want to risk spending the key by activating the third bundle a second time and simply wasting the key.
Per the Steam article on the subject
Extra copies are not given in most cases.
Only titles purchased in special promotions as outlined below are eligible for giftable extra copies
There has also been some discussion in gaming chat about the ethics of giving away extra humble bundle keys, as the site says the keys are for your sole use.
"Our Steam keys do not generate giftable copies on Steam. Please consider your bundle a "unit" and not to share copies of the games or Steam keys from it.
If you'd like to give a copy of the games away, we have a system in place that allows you to buy a gift key from www.humblebundle.com so your friends can have access to direct downloads as well as any Steam or other keys we have included.
Please let me know if there's anything else I can help you with.
Thank you for your support"
For non-Humble Bundle trading, I would suggest the meta post related to game trading.
On your personal Humble Bundle page, you'll see this message beneath the Steam keys:
All keys are for your personal use only. Steam will not provide you with extra giftable copies of any bundle games you already own prior to redeeming your bundle key(s).
So no, you are not given extra copies for this. Technically, you're not really allowed to give the keys away as per the "personal use" clause, however, most people interpret this to mean "I can do what I want with them" and do so.