"Exists in Your Account" Problem.

Try Syncing your account

Posted the same as js93583 :)

I've got the same issue, tried syncing manually for 3 days in a row, still the game shows on my list on SteamGifts, but not on Steam.

Which game?

Most likely due to it being a free weekend game.

i belive it's a steam problem cuz, if you go to the store it says that you already own the game, and if you press install it will popup the news in the steam app.

Got the same crap with Sanctum 2 giveways.The fork?

According to SteamStore (not SG), I still own Ace Combat Assault Horizon (since free weekend), and it shows in client library. Button shows "Play", not "purchase" like with preloaded games, even after wiping folder and corresponding appmanifest, but trying to launch it redirects to store. Good thing I don't want the game, but still it's somewhat annoying.

bobofatt (Support) (1 year ago) #15606348
This is a known issue with free weekends and guest passes. All that can be done is for the user to remove any game content they may have on their computer and resync their SteamGifts account. It can take up to two weeks for Steam to update the user's library to reflect they do not own the game, however. Sadly, this is a problem on Steam's end and we can't change it.
To prevent this happening in the future, manually sync your account mid-week, like Tuesday or Wednesday. This will reset the 7-day sync to work on the day of the week you last synced, so it won't sync on the weekend unless you do it manually.

The same will happen to some Shogun 2 DLC.