Before inserting check if post exists:
$exists = get_page_by_title( get_field('fornecedor'), OBJECT, 'fornecedores');
if ( empty( $exists ) ) {
wp_insert_post( array(
'post_status' => 'publish',
'post_type' => 'fornecedores', // post_type which is created posting
'post_title' => get_field('fornecedor'), // Custom field to generate the post type
'post_content' => ''
)
}