URL Rewrite with Web Applications

Hi,

I am running an ASP.NET 4.0 website using IIS7 on Windows Server 2008 R2.

The site has a number of Web Applications under the main domain (~/site1, ~/site2, ~/site3, etc...). Each Web Application is an instance of the same website source folder (D:\Websites\MySite), so you can access the same website at any of the following URLs:

http://www.mydomain.com
http://www.mydomain.com/site1
http://www.mydomain.com/site2
http://www.mydomain.com/site3

The same website is returned each time, but with slightly different content depending on which specific URL is called.

Everything has been working fine for months until I was asked to add some SEO optimizations using URL Rewrite. Every URL Rewrite rule I add produces the following error:

HTTP Error 500.52 - URL Rewrite Module Error.
The page cannot be displayed because an internal server error has occurred.

Config Error:Cannot add duplicate collection entry of type 'rule' with unique key attribute 'name' set to 'AddTrailingSlashRule1'

I have read elswhere that surrounding the <system.webServer> configSection in the web.config file with:

<location path="." inheritInChildApplications="false">

will fix this problem.

However, while it stops the error message from appearing, it also stops the URL Rewrite rules from working in any of the Web Applications (~/site1, ~/site2, ~/site3).

How can I get the URL Rewrite module to work with Web Applications?

Thanks for any help you can offer.