db已成功连接，但我运行了代码，然后在db中出现重复行，为什么？

public class MySQLTest 
{

    public static void main(String[] args) 
    {

        try {
            Connection con=DriverManager.getConnection(url, username, password);
            String sql = "INSERT INTO customer (firstname,lastname) VALUES (?,?)";
            PreparedStatement statement = con.prepareStatement(sql);
            statement.setString(1,"Name");
            statement.setString(2, "LastName");

            int rows = statement.executeUpdate();
            if (rows > 0 ) {
                System.out.println("A row is inserted");

            }
            statement.close();
            con.close();
        }