Weird. I already own this dlc but it allows me to enter. Not entering for obvious reasons, but still, it might be the same for other people so it would be good to check on that.

thanks :D

Much appreciated!

Dear giveaway creator, it seems there are some limitations regarding detecting DLCs and people already owning the DLC can enter a giveaway. Therefore, you must make sure to check whether the winner already has it or not and maybe pick a new winner.
Here is the answer I received from SG support:

Sadly fixing detection of DLC is not something SG admin can do.
We can sync only information visible here:
https://steamcommunity.com/id/2131860468400/games/?tab=all
As Steam does not send list of user owned DLCs via their SteamAPI - we are unable to sync them.
Creator can still check if winner already have DLC they won on their Steam account - but they need to add them on Steam and try to gift them the DLC. If someone owns it - it will show "already owns it" message.

ty