Checking validity of keys

I think you can do that, if it's used it will say that it is a duplicate, if not it will say that you own it.

No, there is no way to check if key works or not, without actually activating.

No. If you already own the game it will simply say you already own it, regardless of whether or not it's been used

if you already own it on steam. then if you try to activate it, it will say that you own it and the key wont activate.

I made a thread about this a few months ago and it turns out that you can't. The only way is to contact steam support or the place you bought it from, that's the only info I have. Otherwise you can't check it on Steam because it will be activated as soon as you use it.

Feel free to try.
Here are 3 keys that have been used by me:
eden*
...

What will they say if you already own the game?

If you enter a key for a game you own you will get the message that you already own the game. This is what will happen whether the key is a used one or a new one. I've tried to ninja some keys enough times to know this.