Problem with Cannot add duplicate collection entry of type 'add' with unique key attribute 'name'

Problem with Cannot add duplicate collection entry of type 'add' with unique key attribute 'name' set to.....

 If I m not wrong your problem may be looks something like this.. 

The Problem Is Not That You Have Done Something Wrong With Code...,
The Problem Is Because You Have Done Something Like This...

1)You Published Your Project In a Folder
2)Then You Created Final Virtual Directory In The Same Folder...So What Happens Usually Is ......There Are TWO web configs under the wwwroot folder...and you better knows that TWO SWORD CAN NOT BE PLACE IN SINGLE SCABBARD.
3)So Just Remove The External WebConfig....by means of external i mean the first web config before you have created virtual directory.
Let Me Know If You Face Further Error.