Integrity key

I tried even if the key has already been used tells you that you already have the game in inventory

Actually, in my experience, Steam only tells you already own it, regardless the fact that the key has been used or not. Maybe it's different now.

For what I've seen, it works like that:
1 - it checks the key: valid (existing in their database) or not. If it's an invalid key, uplay for example, process stops here with an error message.
2 - it now knows for what game the key is. It checks if you already have that game. If yes, it stops here and tells you why.
3 - only at this third step, it checks if the key is already redeemed or not.
And yes, I agree it would be way better if step 3 was done during step 1 :)

Nope, wether the key has been used or not, you will get the exact same message about already owning the game.