using System.Collections.Generic;

using System.Linq;

using System;

public class Program

{

    record A(string Prop1, int Prop2);

    public static void Main()

    {

        var list = new List<A>() {

            new A("A", 1),

            new A("A", 2),

            new A("B", 2)

        };

        var groupedList = list.GroupBy(a => a.Prop1);

​

        foreach (var grouping in groupedList)

        {

            if (grouping.ToList().Count > 1)

            {

                Console.WriteLine($"More than one element {string.Join(", ", grouping.ToList())}");

            }

        }

    }

}