IIS Error: Cannot add duplicate collection entry of type 'add' with unique key attribute 'name' set to 'AuthenticationModule'

I am getting a duplicate key error but I am not seeing where the duplicate is?

I see only one entry in my web.config file for that key. Where else would that duplicate exist?

Error Info...

Detailed Error Information:
Module     IIS Web Core
Notification       BeginRequest
Handler    Not yet determined
Error Code     0x800700b7
Config Error       Cannot add duplicate collection entry of type 'add' with unique key attribute 'name' set to 'AuthenticationModule'
Config File    <path info>...\Office\web.config
Requested URL      http://<url info>/Home.aspx
Physical Path      <path info>...\Home.aspx
Logon Method       Not yet determined
Logon User     Not yet determined