Hare Apparent is suffering the same problem in Brawl.

Same with nazgul

Also marking decks with multiple copies of ***** against Humanity and Persistent Petitioners as invalid

NAZGUL MULTIPLE COPY ERROR....I CAN'T PLAY WITH MY BROWL RISOLVETEEEEEEEEEEEEEEEEEEEEEEE

Same here! Even eliminated my decks with Hare Apparent

I's also noticed this issue with my ***** deck, rather annoying that this is happening!

I have the same issue with my persistent petitioners deck. not working after the update.

I have the same issue: my Brawl deck is flagged as invalid if I include more than one copy of Nazgûl.

Same issue but in my case it's with my 39 copies of Rat Colony.

I'm having the same issue but with copies of Shadowborn Apostle, which says a deck can have any number of copies. I would assume it's the same with similar cards like Relentless Rats or Persistent Petitioners where they have a specific rule that trumps the normal singleton rules.

I've noticed this same issue, today, as well! I'm glad I'm not the only one who noticed just today.

Yeah this has happened for me too. I was playing it earlier today before the update but now isn't working saying there are multiple copies of a card in the deck so therefore invalid. Nazgul is the only multiple copies of a card in the deck apart from basic lands!