How do you use LINQ to find the duplicate of a specific property?

Question

Customer customerOne = new Customer(\"John\", \"Doe\");
Customer customerTwo = new Customer(\"Super\", \"Man\");
Customer customerThree = new Customer(\"Crazy\", \"Guy\"         


        

Answer 1

 customers.GroupBy(c => c.LastName).Where(g => g.Skip(1).Any()).SelectMany(c => c)

or with LINQ syntax:

        var q = from c in customers
                group c by c.LastName into g
                where g.Skip(1).Any()
                from c in g
                select c;

Answer 2

    var result = from c in customers
                 join c2 in customers on c.LastName equals c2.LastName
                 where c != c2
                 select c;