prepared statement already exists

Hi! In a function to insert rows into a table, I keep
getting ERROR: prepared statement "updateplan" already
exists. If any ideas; thanks.

CREATE OR REPLACE FUNCTION testPreparedStatement()
RETURNS SETOF FLOAT AS $$

DECLARE
tryint1 int4:=1 ;
tryint2 int4:=2 ;
BEGIN

PREPARE updatePlan ( int4, int4 ) AS INSERT INTO
testtable VALUES ( $1, $2 );

EXECUTE updatePlan ( tryint1, tryint2 ) ;

RETURN NEXT tryint1;
END;
$$
LANGUAGE 'plpgsql' VOLATILE;

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