Does using a key for a game I already own invalidate the key?

yes it is still valid

Thanks.

Have created the giveaway with a warning in the description.

http://www.steamgifts.com/giveaway/1HmQu/trauma

If that small chance occurs and the key is invalid its your responsibility not the entrants.

If the game says "Product already owned" the key is still valid. Ofc this is only true if the key was still valid before you used it. If it was already used then steam will stell tell you "Product already owned",although the key is used already. However,there are some cases were a key can be activated although you own the game,but then again you don't get "product already owned",you just get a normal activation window.

It should be valid in all probability.I have done it many a times myself.

If you aren't shown the "game X has been added to your account", then the key is still valid :) (if, of course, it wasn't already used before)

nope