SharePoint Web Application Creation Error

 SharePoint 2010 Web Application creation error: "There is a duplicate 'Reporting Services/Data Extensions' section defined"
I was trying to create a new Web Application on my SharePoint farm and as soon as it was created, I got the following error:

When I try to browse to the site, I see a very helpful error.



Looks like a small problem.  So I went into the Web.Config file and removed that duplicate entry.

Problem was that now I got a "Server Error 500" when I tried to access the Web App's Site Collection.

I googled, fiddled, interogated logs.  All to know avail.

Eventually, in desparation, I decided to uninstall SharePoint and try again. 

Unfortunately, this time I got the above error as soon as I created the Central Admin Database.

So what was going ON?

In my case, I found that this was an issue with the Supplemental .Config file that I had created.

I had never created one of these before but, as per Microsoft Best Practice, the SharePoint Web.Config file should not be editied directly.  Changes should be made in a custom file, stored in %CommonProgramFiles%\Microsoft Shared\web server extensions\14\CONFIG

I took a loko in this directory and what should I see but not only my Supplemental file, but a very suspicious looking file called "Webconfig.rs". 

So, I removed my own Supplemental file and it resolved the issue.

I haven't figured out the exact cause of this issue yet. I suspect that as the Web Config file is updated when a new Web App is created was getting muddled.  Most probably caused by some malformed XML in my own file. 

The good news is that the problem is fixed and I can continue my install.