Games that come with an extra key

Space farmers
http://store.steampowered.com/app/271570/

ibb & obb
http://store.steampowered.com/app/95400/

The Ship
http://store.steampowered.com/app/2400/

Yet Another Zombie Defense:
http://store.steampowered.com/app/270550/ (I'm not 100% sure about this one, but I think it does give you an extra copy)

Crash Drive 2
http://store.steampowered.com/app/343440/

i think they fixed the ship that it doesnt give an extra key anymore though.
Crash drive 2 gives u an extra copy

TRISTOY
http://store.steampowered.com/app/303260/

Achron
http://store.steampowered.com/app/109700/

Probably doesn't meet the criteria you're looking for, but activating my Ziggurat key gave me a gift copy of Little Racers STREET

If you buy Half Minute Hero 2 you get the first 1 too.

Gravity Ghost comes with another copy
http://store.steampowered.com/app/251490/

Takedown Red Sabre did - not sure if it still does.

Interstellar Marines - Spearhead Edition
store.steampowered.com/app/236370

Not sure if they still apply but I got an extra key from Outland and Divinity Original Sin Collectors Edition

I know it's not quite the same, but:

When I bought Hack n' Slash, I got a copy of Spacebase DS-9

Also, when I activated a Talos Principle Key, I think I got a copy of Serious Sam 3.

When I got Heavy Bullets from humble they gave me an extra copy in my inventory

Forget my other post :P

Space Farmers: http://store.steampowered.com/app/271570/

About This Game: This is a Double Pack - you'll get 2 copies so you can give one to a friend. Space Farmers is a co-op game only and has no single player mode.

Secret Ponchos: http://store.steampowered.com/app/265750/

Buy Secret Ponchos - Includes a free gift copy of Secret Ponchos!

https://steamdb.info/search/?a=sub_keynames&keyname=12&operator=1&keyvalue=

That explains why I see so many of those on here.