They aren't forcing any single property to be a duplex. Instead they are preventing a ban on allowing the property to be a duplex. The property owner can still choose to build a single family home at their discretion.

The comparable car example would be preventing a local ordnance that bans minivans. The car buyer can choose a car or minivan based on their discretion if such a ban is prevented. Some localities don't like minivans for whatever reason and would like to ban them, thereby forcing everyone to buy cars.