Due to my addiction to crusader kings, I'm waiting for the dlc and europa universalis IV to go on sale. Annoyingly, the complete collection is on sale on humble for cheaper than even half the dlc cost but I don't know if you can even get a complete collection if you have the game.
Anyone know how this works?
My experience differs from Vaendryl's there. Bundles I bought which contained duplicates of games I already owned just added the missing ones. If there is any new DLC of an already owned game included, it just gets installed on top of the existing installation.
Maybe the behaviour depends on the game and how the developer publishes it? "Normal" steam-style distribution vs. completely different steam application IDs for bundle?
It's probably safe to say that you won't lose anything, tough. And if you do, there is always Steam Support to sort that stuff out.