cannot add duplicate collection entry of type 'add' with unique key attribute 'name' set to 'RewriteConfig'

This error can occur in certain circumstances where IIS is not properly configured or is in a special mode often caused by configuration with Plesk and other automated installers.

To correct this, go to the line indicated in the web.config and immediately above it add the following line:

<remove name="RewriteConfig"/>

Save the file and everything should work again.

This occurs because while the application pool is in integrated mode, it's also processing the classic mode settings in the file incorrectly.