I can try to explain again: normally you won't see a game or be able to enter a giveaway because Steamgifts talks to Steam. It uses what's called an API, or application programming interface, to ask Steam if your account includes a game in your library. This works well, except in some cases, where it doesn't return an answer. As a result, even though I own Don't Starve: Reign of Giants, it looks like I don't - it showed in my list of giveaways and on this page, I see a green "Enter Giveaway" link instead of a red message saying the DLC already exists in my library.

People often rely on Steamgifts hiding games you already own, especially when you have a game with many DLC. If the giver doesn't check before the key's given out, the winner may try to redeem with and will waste the key and end up with "multiple wins" against their record (blocking them from some giveaways). To avoid that, the giver needs to know there's a risk that winners might win DLC that they already own, hence my warning.

Same.

Wouldn't have been a big deal but an inconvenience for the giver.