Why Duplicate posts are being added while fetching data from RETS server?

I am trying to add posts in wordpress from rets server using PHRETS. Unfortunately duplicate posts are being added. I have used the WP Query to check the existing post using meta key and value. This code is running well when I am trying to add 10 or 50 posts but when I set the limit to 4000 it start adding duplicate posts. I have run this code so many time and flushing the database so many time. Here is a code sample:

$query = "(922=MIAMI), (246=A)";
$search = $rets->SearchQuery("Property", $class, $query, array("SystemName" => 1, 'Limit' =>4550));

if ($rets->NumRows($search) > 0) {
    $fields_order = $rets->SearchGetFields($search);

    while ($record = $rets->FetchRow($search)) {
        foreach ($fields_order as $fo) { 
            if ($fo == 'sysid') { $systemid = $record[$fo] ; }
            if ($fo == '881') { $saddress = isset($record[$fo]) ? $record[$fo] : ""; }
            if ($fo == '214') { $sremarks = isset($record[$fo]) ? $record[$fo] : ""; }
        }

        $porpertytitle = $saddress;

        $args = array(
            'numberposts' => -1,
            'post_type' => 'property',
            'post_status' => 'publish',
            'meta_key' =>'sysid',
            'meta_value' => $systemid
        );

        $the_query = new WP_Query($args);

        if($the_query->have_posts()) {
            while ($the_query->have_posts()) {
                $the_query->the_post();
                unset($systemid);
                unset($args);
            }
        } else {
            $my_listing = array(
                'post_title' => $porpertytitle,
                'post_type' => 'property',
                'post_content' => $sremarks,
                'post_status' => 'publish',
            );

            $listing_post_id = wp_insert_post($my_listing);

            if($listing_post_id > 0) {
                update_post_meta($listing_post_id, 'sysid', $systemid);
            }

            unset($systemid);
            unset($args);
            unset($listing_post_id);
        }
        wp_reset_postdata();
    }
}

Sorry for this long code. But I have also tried unset the variables after every loop but not working.