Humble gift link claimed but I already own the game

Then again my brother probably could have told me what the game was instead of just sending it ;D

I didn't actually know that it was possible to enter giveaways for a game you already own.

Typically it's an issue with DLCs (when obtained from a source other than SG), as the Steam API doesn't properly send ownership information for those, and thus they need to be manually hidden (by clicking the 'eye' icon, which is next to the giveaway title on the giveaway list, or at the top-center of the giveaway on the giveaway page).
It can also occur if you've activated a game which you obtained elsewhere, but haven't yet resynced your Steam account with SG (which you can manually do here).
It used to also be a problem with SG wins as well, but SG now flags out games for a duration after you've won them through SG, though you should still be able to deliberately bypass that (which is useful if the first GA creator was unable to provide the game you won [or if you want to try aiming for a permanent vacation from the site by abusing giveaway entry] ).

As mentioned above I did not receive in a giveaway but good to know.

If it's not giveaway related, you can also simply just message Humble to reset the link. That's something which is more useful for when you accidentally toggled key reveal where you intended to get a gift link, though, since- as I noted- you can just manually get the key still even after toggling gift link (and, due to Humble's recent crackdown on grey market site usage of their keys, I'm not entirely sure they'll reset gift links anymore anyway).