System.ArgumentException while updating Model from Database

Our Application is an MVC Application. We are using Entity Framework. When I am updating the model to add a table from the database. I am recieving an exception and it says

"An exception of type 'System.ArgumentException' occured while attempting to update from the database. The exception message is: 'An entry with the same key already exists'."

I am not able to figure out what is the problem. Can I have a solution to get over the problem.