System.Web.WebPages.dll occurs twice

Error 4 The type 'System.Web.Mvc.ModelClientValidationRule' exists in both 'c:\Program Files (x86)\Microsoft ASP.NET\ASP.NET MVC 3\Assemblies\System.Web.Mvc.dll' and 'c:\Program Files (x86)\Microsoft ASP.NET\ASP.NET Web Pages\v2.0\Assemblies\System.Web.WebPages.dll' E:\ASPNet\MvcEnrollmentSystem\MvcEnrollmentSystem\Models\AccountModels.cs 274 32 MvcEnrollmentSystem