Have you ever get the following error when you run Central Administration in SharePoint Server? I hope you haven’t yet and make yourself ready to fix this error for future 
Config Error There is a duplicate 'system.web.extensions/scripting/scriptResourceHandler'section defined
Problem: I have installed SharePoint 2010 in a server and recently I got this error in my server when I tried to run central administration. I was stuck here to resolve this issue as it was my staging server and I can not re-build everything from scratch. How I resolved this problem?
Solution: I started to run SharePoint Configuration Wizard to Fix this but I got the following Error
Failed to provision the SharePoint Central Administration Web Application.
An exception of type System.Runtime.InteropServices.COMException was thrown. Additional exception information: Filename: \\?\C:\inetpub\wwwroot\wss\VirtualDirectories\46824\web.config
Line number: 25
Error: There is a duplicate 'system.web.extensions/scripting/scriptResourceHandler' section defined
This issue occurred due to running the SharePoint Application pool under .NET framework 4.0, however, the application is compiled with .NET framework 3.5. SharePoint 2010 is running under .NET framework 3.5. So, in this case, please make sure the application pool is running under .NET framework 3.5.
Details:
http://mssharepointtips.com/tip.asp?id=1172&page=1
Note: DO not remember to re-start IIS after changing .NET framework version to V2.0.50727.