Ошибка: The type 'System.Web.UI.UpdatePanel' exists in Framework 2 and 3.5

Error	103	
The type 'System.Web.UI.UpdatePanel' exists in both 
'c:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\project1\b2f9b279\fadf0830\assembly\dl3\6f371362\337de8b1_bde2ca01\System.Web.Extensions.DLL' 
and 
'c:\WINDOWS\assembly\GAC_MSIL\System.Web.Extensions\3.5.0.0__31bf3856ad364e35\System.Web.Extensions.dll'	
D:\project1\Pages\swer1.aspx	220