An item with the same key has already been added

Most likely, you have model which contains the same property twice. Perhaps you are using new to hide the base property.

Solution is to override the property or use another name.

If you share your model, we would be able to elaborate more.

I had the same problem and this is how I solved it. I had a duplicate property with the same name in my ViewModel. One Property was in BaseViewModel and another is in derived Model.

public class BaseviewModel{
  public int UserId { get; set; }
}


 public class Model : BaseViewModel
 {
     public int UserId { get; set; }
 }

I changed that to

public class BaseviewModel{
   public int UserId { get; set; }
}


public class Model : BaseViewModel
{
    public int User_Id { get; set; }
}

Now it is working fine.

I had 2 model properties like this

public int LinkId {get;set;}
public int LinkID {get;set;}

it is strange that it threw this error for these 2 haha..