مشکل با scriptmanager

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اموقع اجرا این Error می دهد
Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.

Compiler Error Message: CS0433: The type 'System.Web.UI.ScriptManager' exists in both 'c:\WINDOWS\assembly\GAC_MSIL\System.Web.Extension s\1.0.61025.0__31bf3856ad364e35\System.Web.Extensi ons.dll' and 'c:\WINDOWS\assembly\GAC_MSIL\System.Web.Extension s\3.5.0.0__31bf3856ad364e35\System.Web.Extensions. dll'