Bans are duplicated to keep champs from being permabanned
Sounds crazy, but I think this is what was going through Riots mind.
They think players will do things like take a gamble and say "Alright, I think the other team will ban Lulu, so I'll ban a different champ so something else is blocked as well."
This creates a risk on whether or not the other team is thinking the same thing or not, leaving the possibility of no one banning Lulu.