Battletag: Marth#2571 Region: EU Trade Only?: Yes, you Go first
Blizzard can simply make so you can't add duplicates to your deck, when making it, in the collection manager. Just like you can't add Invoke cards without Galakrond.
That's simply not true, you can add invoke cards without having Galakrond in your deck. Furthermore the last thing they want to do is restrict freedom around the deck building process.
They specifically said that the thought process they had in mind regarding Highlander cards was that players should be able to "counter" it. They mentionned Albatross as an example to that (bombs also fit this idea). They may even design futur cards for this purpose.
Still, I agree it's a bit of bummer to know that non highlander decks can benefit from Zephrys or Alex (OTK Demon hunter and Galkrond Warlock for example).