Registered game I already had?

The game key is still valid if that's the case. :)

yep you can create a giveaway. The key will still work

Most of the time your game key will still be fine (when you try to activate, Steam will let you know that you already owned the product, and will not 'use up' your gamekey).

HOWEVER there are some games that Steam can double register, due to the keys activating different packages for the same game. I wasted a key like this recently (I think it was for 'The Red Solstice').

I made mistake like that with Saints Row 2.

Support ticket and some time (week?) later my key was valid again.

Depends on if it said <game> has been activated, or if it said <game> already exists in your library, hit next to install.

The first (can happen with GOTY/packages) means it's no longer valid, the second means it wasn't used.

If you got already owned message after entering the key then the key should still be valid, but if you get duplicate code message then it was already used.