提示这种错误，为什么？？

//编译提示错误，为什么 1 类型“Employee”已经包含“salary”的定义 C:\Documents and Settings\liuminggui\My Documents\Visual Studio 2005\Projects\ConsoleApplication1\ConsoleApplication1\val_by_ref.cs 152 16 ConsoleApplication1
public class Employee
{
   private  int salary;    public int salary
    {
        get
        {
            return salary;
        }
        set
        {
            salary = value;        }
    }
}
    public class Taxation
    {
        public static void Calculate(Employee employee)
        {
            employee.salary = employee.salary - (employee.salary * 22 / 100);        }
    }    class Calculate
    {
        static void Main(string[] args)
        {
            Employee employee = new Employee();            employee.salary = 40000;
            Console.WriteLine("Gross  salary  before  tax-{0}", employee.salary);            Taxation.Calculate(employee);            Console.WriteLine("Net salary after  tax -{0}", employee.salary);
        }
    }