dblink question about duplicate connection

Dear all,
i have a question about dblink connection.I use it in a trigger function :

CREATE OR REPLACE FUNCTION setuser()
RETURNS trigger AS
$BODY$

DECLARE

BEGIN
IF (TG_OP = 'INSERT') THEN

PERFORM dblink_connect('conn_test', 'dbname=database_test');

PERFORM dblink_exec('conn_test','insert into users
......

Somentimes happend that an error accours in insert data and when i try to run
new insert posgresql return and error like duplicate connection.
I would like to change my functions in this way:

CREATE OR REPLACE FUNCTION setuser()
RETURNS trigger AS
$BODY$

DECLARE

BEGIN
IF (TG_OP = 'INSERT') THEN

If 'conn_test' exist PERFORM dblink_disconnect('conn_test');
else

PERFORM dblink_connect('conn_test', 'dbname=database_test');

PERFORM dblink_exec('conn_test','insert into users

How can write my new function?

Thanks

Francesco Boccacci