When opening packs, as of the most recent Hearthstone update, it is impossible to open a copy of a legendary card that you already own until you own every legendary. I was wondering if this also applies to the Prologue reward in the Frozen Throne solo adventure. You are awarded with a Hero card for completing the mission: can you get a copy of one that you already own?
No this can't happen.
This is a reddit comment from a Team5 Member:
Not to worry! Non-pack rewards, such as those offered from the Welcome Bundle, Arena, or Heroic Tavern Brawl will follow the same duplication rules that will apply to opening card packs. The same will be true for the upcoming Prologue reward. You won't need to obtain them in a specific order to avoid getting a duplicate Legendary.