Normalizing removes data redundancy. This adds data redundancy.
When I design a database structure, it's common to start with the most normalized representation possible. And then to denormalize the minimum necessary for performance reasons -- duplicating rows and/or columns just like here so certain data can be retrieved more quickly, whenever indexes aren't powerful or featured enough.
create table purchase_order (
id int primary key,
ordered_on timestamptz not null,
customer_id int not null references customer,
canceled_on timestamptz
);
create table purchase_order (
id int primary key,
ordered_on timestamptz not null,
customer_id int not null references customer
);
create table order_cancelation (
order_id int primary key references purchase_order,
canceled_on timestamptz not null
);