Hmm, again on this matter: maybe you’ve already opened all legendaries and thus got a duplicate? I’m not sure how it should be technically possible otherwise, since, according to the support article:
You will not receive more than one copy of a Legendary card, until you have owned all the Legendary cards in that card expansion.
There’s also been another issue: Unable to reroll? - #3 by SparkyElf-2852 — however, I’m not sure if that applies in your case.
PS
(Updated and edited)