Most times, our clients want a list that is "one per household". Generally that means they want "one per address" so there aren't duplicate addresses being mailed. But what about when household isn't so clearly defined. What if there are unrelated roommates? What about grown adults living with parents or grandparents? Should each of them get their own mail piece?
Most mailing list compilers do think of one per household as being one per common last name. If there are roomate situations or other configurations within the home, there will likely be duplicates to the address, resulting in the need to remove duplicates prior to mailing, if in fact you only wanted one per address.
There is not any one answer to this - so, each mailing campaign should carefully consider this. It's an important distinction to make when ordering the data.