using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    class Test

    {

        public int Id { get; set; }

        public int Number { get; set; }

        public int ContactId { get; set; }

    }

    public static void Main()

    {

        var data = new List<Test>

        {

            new Test { Id = 1, Number = 1234, ContactId = 5 },

            new Test { Id = 2, Number = 9873, ContactId = 6 },

            new Test { Id = 3, Number = 1234, ContactId = 7 },

            new Test { Id = 4, Number = 9873, ContactId = 6 },

        };

        var results = data.GroupBy(number => number.Number)

                .Where(number => number.Count() > 1)

                .Select(number => new

                {

                    Number = number.Key,

                    Count = number.GroupBy(contactId => contactId.ContactId).Count(x => x.Count() == 1)

                })

                .Where(x => x.Count > 0).ToList();

​

        foreach(var item in results)

        {

            Console.WriteLine("Number: {0}, Count: {1}", item.Number, item.Count);

        }

    }

}