An item with the same key has already been added. .NET MVC3

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MVC3

entity-framework

I have dropped and added a new .edmx file to my solution which hascertain fields.
when i try to run the appplication i try to fetch some data from the data base

it gives exception.
"An item with the same key has already been added."

in edmx designer.cs file
base.CreateObjectSet<Object>("xyz");

Any Suggestion Plz help!