I get that there is unique cards, like allies, and there are other cards that come x1 in the core, but can i build my deck with several copies coming from several cores? (Asking for "ancestral knowledge" especificaly, and posible combos or strategies with 3 of it)

I ask because the coreplay says you must use every hero card if i got it right

No. The Hero card sets are (currently always 15 cards) that are numbered. You need to have 1, and only 1, of each number in your deck. Cards that appear multiple times in the set have different numbers.

Thanks! So i got to build my decks only with 1 core and expansion, that's great news!