http://msdn.microsoft.com/en-us/library/bb514115.aspx 

Removing Unused Usings

The Remove Unused Usings option in the Visual Studio user interface removes using directives, using aliases, and extern aliases that are not used in the source code. There are two ways to call the operation:

• Main Menu - On the Edit menu, point to IntelliSense, point to Organize Usings, and then click Remove Unused Usings.

• Context Menu - Right-click anywhere inside the code editor, point to Organize Usings, and then click Remove Unused Usings.

  Note
  If you perform Remove Unused Usings on source code that does not build, some required using directives may be removed.

The following example shows the outcome of performing Remove Unused Usings on source code.

using System;
using System.Linq;
using System.Collections.Generic;
using System.Text;
using System;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("test");
        }
    }
}

using System;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("test");
        }
    }
}

In the previous example, only System is used later in the source code. The other using directives, including the duplicate System using directive, are removed.