How about official information from 25.0 Patch Notes instead? 
New Feature: Improved Duplicate Card Handling
We often include specific non-disenchantable cards as part of Reward Tracks, Events, Achievements, and Shop purchases. Starting with 25.0, if you claim one of these, but already own a lower quality copy that’s disenchantable, you’ll get a popup that offers to re-roll the lower quality copy to a different card of the same rarity from that card set.
For example, Diamond Grand Magister Rommath is on the March of the Lich King Tavern Pass. If you already own a Golden Rommath when you claim the Diamond, you’ll immediately get a popup offering to reroll Golden Rommath into a different March of the Lich King Golden Legendary (which, of course, respects duplicate protection). This feature also applies when you find a Signature card in a pack!
Note the last sentence.
However, as indicated above, there’s been apparently an issue with it.