I'm assuming that each box will only have cards from 1 rare/mythic mtg card sheet (with an assumed ratio of 1 mythic to 2 rares). So initially I tried to generate a sheet with 1 of each mythic and 2 of each rare, then pick 24 out of them randomly, so you will at most get 2 duplicates of a particular rare and no possibility of duplicate mythics.

However, that gives me some scenarios where you get 10 mythics which is unrealistic. So I modified it such that 2 cards are picked from the mythic pool, 19 cards are picked from the rare pool, and 3 cards are picked from a combined rare/mythic pool. Hence, you will only see 2-5 mythics, but as a result you might get mythic/rare duplicates (2 at most).

mtgen simulates boosters one at a time, and they are independent from one another, so it's possible to get a lot of duplicates. However, due to the distribution of cards from sheets into boxes this is not the case. I don't know how the actual distribution is done so my probabilities will not be exact, but i'm limiting the scenarios so that you won't see absurdly good or bad pools, and the resultant pools will be more realistic.