I’ve encountered this issue 2 separate times now since Whizbang launched. Since launch, I’ve opened 2 Signature legendaries (1 from a Whizbang golden pack and 1 from a Classic pack) that duplicate a disenchantable legendary I already own:
• In neither case was I offered the option to reroll the disenchantable version into another legendary from the same expansion.
• I don’t yet own all the craftable legendaries from these expansions, so I now have duplicates without having a complete collection.
• I haven’t dusted any of these missing legendaries, so it’s not the case that I previously owned them.
Details:
CASE 1 <<
From my Whizbang mega-bundle packs, I opened a normal (non-golden) version of the craftable Rogue legendary Sonya Waterdancer. Then in the golden packs from the mega-bundle, I opened a Signature version of Sonya Waterdancer.
At that point, I was still missing 16 legendaries from the Whizbang set. But I wasn’t offered the option to reroll my normal version of Sonya Waterdancer into another Whizbang legendary.
As of today, I’m still missing 2 legendaries from the Whizbang set (Inventor Boom and Galactic Projection Orb). If I were able to reroll my normal version of Sonya Waterdancer — which, per the HS Reroll system (outlined here: https://us.battle.net/support/en/article/325007), I believe I should have been offered the option to do — my understanding is that I would reroll into one of those two missing legendaries.
CASE 2 <<
I purchased 2 Whizbang’s Workshop bundles (Ranks 1 and 2) that contained both Whizbang packs and Classic packs. From one of those Classic packs, I opened a Signature version of the Festival of Legends Warrior legendary Rock Master Voone.
I already own the normal, disenchantable version of Voone. But I wasn’t offered the option to reroll the normal version into another legendary from Festival of Legends.
I’m still missing the Hunter legendary Stranglethorn Heart from Festival of Legends. (I never previously owned Stranglethorn Heart, so I never dusted it.) My understanding is that I should have been able to reroll Voone, and it should’ve rerolled into Stranglethorn Heart.