using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    class MyEntity{

        public int Id { get; set; }

        public int Number { get; set; }

        public int ContactId { get; set; }

    }

    public static void Main()

    {

        var data = new List<MyEntity>

        {

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

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

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

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

        };

        var results = data.Where(x => data.Any(y => y.Id != x.Id && y.Number == x.Number && y.ContactId != x.ContactId))

            .GroupBy(x => x.Number)

            .Select(grp => new { Number = grp.Key, Count = grp.Count() })

            .ToList();

​

        foreach(var item in results)

        {

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

        }

    }

}