Need advice, reroll or not to reroll, that's the question.

You should be able to by going to their Steam profile, and going into their games section. The list normally includes DLCs. This assumes their games list is public... and that Steam shows all the DLC...

" finally is there a possibility to check out how many of DLCs do they own.. I don't really mind to give it to winner who owns only base game without dlcs..."
Nope, and not going to happen until Valve get their shit together and make sure the APIs cover DLCs porperly.

"So apparently looks like both of my winners (of Tropico 4 Collector's Edition) already owns the game"
That'd be enough reason to request a re-roll for me. But then, I'm kind of resistant to givign away packs because of DLC's instead of base game. (Although, for Bethesda stuff I'd make an exception, since for them complete packs are much cheaper than individal DLCs during sales)

Can't you see that on the DLC store page if you add them?

EDIT: That appears to be the way to check, i saw on the DLC page that a certain friend had it and when i searched their games list no DLC was there, so thats one way to check it, not sure if theres a better one.

Last I heard if the winner owns some of a pack or bundle then it is up to you to decide if it is too much and to ask for a re-roll.

Similar discussion on the topic

If you add them on Steam, you can see on the store page of each DLC which of your friends own them. (A bit tedious bu at least it gies you the info)

And if they already have the whole thing, Steam won't even let you gift the game directly to them. (And that can easily bee screenshotted to ask for a re-roll)

Read the FAQ, Ctrl+F "rules for entering to win Steam packs"
The way I interpret it SG will comply to a reroll, but asking or not for one is your choice alone.
There is no way to check through the API what DLC someone owns AFAIK, and some do show in the All Games list but most do not, for Tropico 4 it's the latter.

You can ask for a reroll if you feel like they own the majority of the bundle - Tropico have lots of DLCs, but as far as I know, quite minor ones.
If you bothered to ask us this question that means you want to reroll it :) submit a ticket, and we'll see if support allows it or not

Considering that Tropico Collectors Edition had a price glitch a while back (it was like $0.99 for 4) then I'd probably reroll especially considering it's highly likely they own the base game.

If not you can ask them if they own any DLC before sending.

I bought Tropico 4 Bundle during a glich, have all DLCs and still can entry for CE, so i assume that they have all DLC. Reroll.

Everyone thanks for help :)