Duplicate rows

I have the following expression working in that the process.fluid_id is transfereed to pipe.fluid_id when the column - process.contain has a value of 'ip'.
There is no transfer when the contain column holds other values. Success - so far.

How do I keep the table pipe from being populated with duplicate rows? Among other reasons not to have duplicate rows, I want to make pipe.fluid_id a primary key.

Bob

CREATE TABLE pipe ( fluid_id int4 NOT NULL);
CREATE TABLE process( fluid_id int4 NOT NULL, process varchar, contain varchar) ;

create or replace function base() returns trigger as $$
DECLARE
myrow RECORD;
BEGIN

for myrow in select * from process where contain = 'ip' loop
insert into pipe(fluid_id) values (myrow.fluid_id);
if not found then
do nothing ;

end if;
end loop;
return NULL;
END;
$$ language plpgsql;

create trigger trig1 after insert on process
for each row execute procedure base();

insert into process (fluid_id, process, contain)
values ('1', 'water3', 'ip');