You can apparently use extras in the brawl which i find rather amusing
ive linked to the reddit post i made since i dont chat as much as i used to on here and lost my right to show image links
https://www.reddit.com/r/hearthstone/comments/k1f548/so_fun_fact_about_the_new_brawl_you_can_use/
Why wouldn’t you be able to?
I built a Rogue deck using only weapon synergy cards, won pretty easily.
let me be more clear, i could have 4 copies of a card in my deck if i had the dust or the extra copies i could fill the brawl deck with just 1 card
I wouldnt expect this since it is a rule to have max two copies of a singlecard in your deck and it is not stated by the brawl that this is possible.
I could not find it out on my own because i click the auto-disenchant button once in a while 
Ah I see, should have put 4 deadly poisons in there 
I think Brawls do not follow the rules sometimes. Not sure if it’s a bug or a feature in this case 
Perhaps Blizzard got some new ideas about increasing spending from the whales. Make it possible to add more than 2 copies in a deck so whales would have to craft extra copy of everything.