CoH 2 Already owned, but not and I want to buy

Delete the game

get steam support person to remove the game from your profile, then you can purchase it.

That's because you probably have Company of Heroes 2 Endless Multiplayer
Only way is to create a ticket to Steam Support to remove your game from the list.

If you don`'t want to miss out on the deal you could buy it as a gift and wait till support fixed your problem and then activate the game

Maybe just buy it as gift and then activate it from your inventory. If it will be not possible for some reason you can return it or contact Steam support to deal with this issue. In most cases extended versions of the same product just overrides those you have in the library (lets say Deluxe version with soundtrack included will replace "standard edition" bought previously so probably it will be the same in this scenario).

Buy it as a gift and keep that in your inventory. Then contact support so if that takes longer than the sale you still get the sale price. If they can't help you you can at least get the refund for the gift.

Thanks for your answers :)