So I can enter giveaways for Complete Packs when I already own all the content for them?

You can but it is not a nice thing to do.

I own HiB V and i can enter HiB V giveaways ....

L.A. Noire + DLC Bundle is a different identity then LA Noire Complete Pack

and Magicka + all DLC is and always has been bugged with one of the DLC, it always in the steam store says you don't have one of the DLC when in reality when you try and go buy it, it then says you already own the product

Steamgifts doesn't recognize owned DLCs correctly.

There seems to be a general failure to recognise DLC; most likely a deficiency of the Steam Web API.

Basically, you can, but you shouldn't...

Sancuum is same, also borderlands, I think all games this type..

Although there might be a loophole in the system, the rules say no. What does that tell you?

You can't see from the user's games list if he has DLC or not (apart from few rare cases), so there is no way of telling if the user has DLC already or not.
But if you do enter while having it, that makes you a dick.