Why does Postgres allow duplicate (FK) constraints

Hi,

I was a bit surprised that the following DDL will work:

create table parent (id integer not null primary key);
create table child (id integer not null primary key, pid integer not null);

alter table child
   add constraint fk_child_parent
       foreign key (pid) references parent(id);

alter table child
   add foreign key (pid) references parent(id);


It essentially creates the same foreign key constraint twice.

While I agree that this SQL should not have been written like this in the first place, I wonder why Postgres doesn't
activelyprevent this (like e.g. Oracle). 

Is there a technical reason, or is it simply a matter of "no one cared enough to change this"?

Regards
Thomas