I opened standard pack and got Soulburner Varia from Fall of Ulduar mini-set. Later I bought the whole mini-set for 2000 gold and didn’t receive reroll option for the duplicate Soulburner Varia
Reroll only applies when you get a higher version (golden/diamond/signature)
as far as i know re rolls only hapen when getting an uncraftable version of a card you can dust on your collection