Cause
This problem may occur if there is a duplicate entry for the specified configuration section setting at a higher level in the configuration hierarchy (for example, in a parent site/folder’s web.config or applicationHost.config file). The error message itself points out the location of duplicate entry or entries.
ResolutionExamine the specified configuration file and compare it with its parent applicationHost.config and/or web.config files to check for duplicate entries as suggested by the error message. Either remove the duplicate entry, or make the entry unique. For example, this problem may occur because the ApplicationHost.config file has a duplicate entry for the following code.
To resolve this problem, in the ApplicationHost.config file, delete the duplicate entry for the authorization rule. To do this, follow these steps:
1.Click Start, type Notepad in the Start Search box, right-click Notepad, and then click Run as administrator.
Note If you are prompted for an administrator password or for a confirmation, type the password, or click Continue.
2.On the File menu, click Open, type %windir%System32inetsrvconfigapplicationHost.config in the File name box, and then click Open.
3.In the ApplicationHost.config file, delete the duplicate entry that resembles the following code.