Huh? That's just a truism, not a reason. Yes, at some point after unlocking the 500 icons, voices and palette swaps, you might feasible get a box with multiple legendaries. That's not a problem or anything, because all it is is rewarding a player who has spent a stupid amount of time (or money, I guess) playing the game and who has to have opened multiple boxes of useless stuff.
Even Wizards of the Coast, one of the worst digital entities in the top-tier of game makers, dropped duplicates because of their inherent feelbads. And that was in a F2P game.
That being said, I don't want to overstate this or anything. Duplicates are a problem I hope they fix, but since it's all cosmetic stuff you're hardly ever going to actually see, it's not a huge deal. Just sort of a stray hair in the 4-course meal that is Overwatch.
It's a problem because without duplicates it's unfair for players that join years after launch who would need 400 boxes to unlock everything, whereas early adopters can have all the legendaries with less than 200.