Beginning with Hearthstone's next expansion pack, you will no longer receive duplicates of cards until you have every single Legendary card from that specific set. This applies whether you have the golden or non-golden version of a card. What's more, you will no longer receive more duplicate cards than you can use in a deck. When you open a new set, you will also receive a Legendary within your first ten packs -- guaranteed. The result of all these changes? Players will likely have more Legendary cards than they have now.
Additionally, Overwatch's game director Jeff Kaplan made some interesting announcements in their latest developer video update. If you've been frustrated by receiving duplicate cosmetic items in loot boxes, that will no longer be a problem. Kaplan says, "One of the things that we're going to do is drastically reduce the rate of duplicates that you'll get out of any loot box." It's not eliminating the issue entirely, but the team hopes that the change will be "immediately evident," and that as a result players will buy many, many more loot boxes.