Howcome you can enter giveaways for games you already own now?

You can't. If you can, you either didn't sync or it's a DLC that's not correctly recognized as owned by the API.
What game is it?

Yes, it happens sometimes. I think it's because some games have multiple steam store IDs.

You can enter for collections and some gold/game of the year editions. Generally all packs are not recognized.

You can if it's in a bundle or collection i guess.

games that require you to enter your age before viewing also dont sync,i have almost entered a few of those myself thinking i did not have them.

I have the same problem and it seems to be the same games all the time.
Take on helicopters
Pacman
Hero of the Kingdom 2

There are also a few more, but I don't remember

I can always enter Blades of Time giveaways even though I already own it.

Yeah I've noticed a few special editions and franchise packs. Darksiders Franchise Pack, Skyrim Legendary Edition, Saints Row Franchise Pack, Blades of Time Limited Edition, Pac-Man Championship Edition DX All You Can Eat Edition and Borderlands GOTY are all ones that I own but can enter giveaways for, my account has been synced numerous times since owning them.