Do Foreign Keys Impose Uniqueness
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.