Winning a game i already got

It is against the rules to use a game that you won here to create a giveaway. You must activate all giveaways you win on your Steam account that is linked to your steamgifts account or you will be suspended.

Steamgifts should prevent you from winning a game you already own in most cases, but it is possible to enter and win a game you already own with certain games. If this happens, post a comment on the giveaway letting the giveaway creator know that you already own the game and ask them to request for a new winner.

You may want to read the FAQ and Guidelines.
Welcome to SG :)

You should check out the FAQ, it should answer most of your questions. Welcome to the site and have fun

You only have 31 games on your account right now, so it should be easy to recognise what you own or do not own. Games that have "steam is learning about this game" in their description generally allow you to enter if you already own them. You shouldn't do it. That's why it's always a good idea to check on Steam if you're not sure if it's the same game you already have before hitting "enter." You can use the eye icon next to games or DLCs that you own to add them to your blocked list. That way, you are less likely to accidentally enter for something you already have. If it still happens accidentally, just ask for a new winner.

I check every game on Steam, not to check if I own it, but to check if it is something interesting to play. If not, I generally hide it. You should be doing this anyway. The added benefit is ensuring I didn't already get it in a bundle and forgot about it.

Hope that helps.