using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<MyClass> myClassObject = new List<MyClass>();

        myClassObject.Add(new MyClass() { BillId = "123", classObj = {} });

        myClassObject.Add(new MyClass() { BillId = "777", classObj = {} });

        myClassObject.Add(new MyClass() { BillId = "999", classObj = {} });

        myClassObject.Add(new MyClass() { BillId = "123", classObj = {} });

        var result = myClassObject

              .GroupBy(x => x.BillId)

              .Where(x => !x.Skip(1).Any())

              .Select(x => x.First());

        Console.WriteLine(string.Join(", ", result.Select(x => x.BillId)));

    }

    public class MyClass

    {        

        public object classObj {get;set;}         

        public string BillId {get;set;}         

    }

}