I know from this question, that giving away already bought games isn't possible in steam. However, I already own a game (Counter Strike in my case), but want to have the other CS games, too. The cheapest way would be to buy the CS-pack, which includes all CS games, including the game I already have. Is it possible to take the games but one and give the duplicate game to a friend as a gift? Or would I need to pay for a game I already have without any advantage?
From the Steam support page on extra copies, in most cases you won't receive extra copies of a game you already own when you buy a bundle that includes that game. There are certain exceptions, mostly relating to Valve's own titles, but Counter Strike is not listed amongst them, so you'll have to buy your friend the game/pack separately.
If you buy the pack as a gift, the giftable game in your Steam inventory will be the entire pack, so you can't split it up.
In some circumstances you will explicitly receive gift copies of games you already own. For example, with the recent Bioshock: Infinite preorder bonuses on Steam, they stated clearly that you would.
Humble Bundle packs and other such things may work slightly differently, because they give you Steam product activation keys (which are not yet bound to a Steam account) rather than Steam gift copies. Humble Bundle packages together games into a single Steam key when possible, but older bundles or games added to the bundle after purchasing may provide individual keys. Note that technically you are not supposed to share any extra keys you have.
You can only gift games which come with separate steamkeys. I don't know if you only get one key/item in your steam inventory if you buy the CS-Pack, but I do know that it's not possible to gift some games from the humble bundle packs. Most of them are combined in one steam-key and therefore "lost", when you activate the key on an account with one of those game activated before.
In case of the CS-Pack I guess that steam offers just one "CS-Pack-Item" in your inventory. Otherwise there would be to much trading inside the steam community - yes even more as it is right now (besides the dota2-keys!) ;)