Fixing Error when build a page/project is Visual Studio 2010

I has getting the following error every time I build the my page or project in Visual Studio Express 2012.
Error 78 (0): error CS1703: An assembly with the same identity 'System.Web.Extensions, Version=, Culture=neutral, has already been imported.

To fix it I goggled all over eventually I figured this out:

In Solution Explorer right click on the root of the project then click "Property Pages" (or Shift+F4)

Then it will display something like this:

Select the assembly that is giving problems and then click "Remove"

;) Problems solved.