I opened Artanis in my regular Dark Beyond Packs, and when I bought the miniset after, it didn’t prompt me to re-roll my legendary as I already owned it. Both are the same quality card.
You are only getting a reroll if a card of higher quality can’t be disenchanted.
Ooof, I figured it would roll a diff legendary. Guess that teaches me to buy the mini-set before opening packs. Thanks for the info. I thought it would just re-roll if you owned any copy!