That is the same collection of games, just bundled weirdly by Amazon originally, Indiegala has plenty of similar bundles for other games. I was told it's not the first time Paradox opts for such dodgy decisions. You do not get the 1st game with dlcs registered to your library, but when you launch King Arthur: Collection (https://steamdb.info/app/24470) which you do get in your library that is exactly what it is. You get the 2nd game with dlcs in library just fine. If you want to play the games, it's exactly the same. If you only want to count what's in your library, well it's not.

Apparently, the reason for this is that King Arthur: Collection originally contained only the 1st game with dlcs but was discontinued and removed from the store and this package includes that collection + 2nd game. It's the 2nd time I get this key and I've launched and confirmed the above, you can find the same in steam community forums for the game. Here's an image of what it adds to the library exactly, with dlcs registered as well. https://imgur.com/z6gVfOK