An item with the same key has already been added

"exceptionType": "System.ArgumentException",
  "message": "An item with the same key has already been added.",  "exceptionType": "System.ArgumentException",
  "message": "An item with the same key has already been added.",

Solution: In your model or .cs file same keys
Ex: public class Student
{
 public int Sid {set; get;};
 public string Name {get; set;}
 public sting Sid {set; get;} //Same Name - here we will get above exception. if we chnage to another name this issue will resolve.

}