I got my battlepass at lvl 82 and got the signature card of “Lady_Deathwhisper”
i had it us an normal legendary ( bought from path of Arthas ) and when i reroll it that you had announced that its dup protected but i got " The_scourge " ( that i have from path of Arthas )
Duplicate protection means that you will not get a duplicate until you have (or had and then disenchanted) all cards of the same rarity and of the same set. The Path of Arthas set has only three Legendaries: Lady Deathwhisper, Frostmourne, and The Scourge.
Are you sure you do not own and never have owned Frostmourne?
i have them all from the path of arthas pack from shop
In that case, the reroll could impossibly have given you a legendary that you do not own yet.