Do Foreign Keys Impose Uniqueness Do FOREIGN KEYs impose the uniqueness of the the referent?
I.e. if I have a foreign key that refers to a UNIQUE constrained column, will the FOREIGN KEY column be unique?
I'm particularly wondering about postgresql.
No, foreign keys are not implicitly unique. And they can't be otherwise it wouldn't be possible to implement a one-to-many relationship, e.g. this:

create table customer 
(
   id integer primary key
);
create table orders 
(
   id integer primary key, 
   customer_id integer not null references customer
);

insert into customer values (1);
insert into orders values (1,1), (2,1), (3,1);


You can however make them unique if you want. This is typically used when modelling a a one-to-one relationship.