Can’t get wp_insert_post to work

That is because you’re trying to define a property that is already defined, query a table with a wrong name, loop through an integer while you can use get_results instead of query method of $wpdb …

I edited the code, hope it will help with your post duplication process:

defined( 'WP_USE_THEMES' ) || define( 'WP_USE_THEMES', false );
require_once('wp-load.php');
global $wpdb;
$sql = "SELECT * FROM {$wpdb->prefix}newposts";
$result = $wpdb->get_results( $sql );  
foreach ( $result as $row ):
    $row = ( array ) $row;
    $post = array(
        'post_title' => $row['post_title'],
        'post_content' => $row['post_content'],
        'post_date' => $row['post_date'],
        'post_date_gmt' => $row['post_date_gmt'],
        'post_status' => 'publish',
        'post_author' => 1,
        'post_category' => array(1)
    );
    wp_insert_post( $post );
    echo "inserted post {$row['post_title']}";
    echo "<br />";
endforeach;