You should be able to buy it. We've looked into the matter and didn't find anything wrong that would explain this, so I'm not sure why it would happen to you.

I do know that the Shadow Warrior version copy gets automatically bound to the account on purchase of the main game. This means that you will get Viscera Cleanup Detail and the Santa's Rampage spin-off, but will not get an extra copy of the Shadow Warrior one upon purchase.

The items are linked in a pack, which makes it easier receive them, instead of in 3 separate parts. So Steam won't let you have a loose key of Shadow Warrior to use as you please.

That's usually how the purchase operates, but you should be able to go ahead with it without issue.

I am having the exact same issue.

http://minimoose.net/error-buy1.png
http://minimoose.net/error-buy2.png

Please Advise.

I also own the Shadow Warrior version of the game.
I don't own the normal version of the game.
When I put these 3 items in my cart:

Viscera Cleanup Detail
Viscera Cleanup Detail: Santa's Rampage
Viscera Cleanup Detail - House of Horror

I don't get any warning notifications.

i resurrect this problem. ich have shadow warrior since 2013 played cleanup a little tried now to get the fullgame but steam said i cant buy because cleanup shadow warrior is part of it and blocked my attempts to buy it. I thought there was a function to buy it anyway but nope, just too many tries warning after removing it and buying the rest of my cart

after my comment i bought only the chrismas edition that was working. Now i tried just for fun to buy the main game. it warned about shadow and santa i said continue and it worked. I hope this will help others

The Christmas Edition is free when you purchase the normal Cleanup Detail. So don't buy that if you want to buy the whole game anyway. Don't know why they did an extra version of that. Almost bought that twice now.

so i tryed to buy Viscera Cleanup Detail but I can't because i own Viscera Cleanup Detail: Shadow Warrior so why is that