using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

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

        list.Add(new MyClass(){ Text = "A", Flag = true , Number = 10});

        list.Add(new MyClass(){ Text = "A", Flag = false , Number = 1});

        list.Add(new MyClass(){ Text = "A", Flag = false , Number = 8});

        list.Add(new MyClass(){ Text = "B", Flag = true , Number = 10});

        var dupText = FindDuplicates<MyClass, string>(list, nameof(MyClass.Text));

        var dupFlag = FindDuplicates<MyClass, bool>(list, nameof(MyClass.Flag));

        var dupNumber = FindDuplicates<MyClass, int>(list, nameof(MyClass.Number));

        list.Dump();

        dupText.Dump();

        dupFlag.Dump();

        dupNumber.Dump();

    }

    public static IEnumerable<TReturn> FindDuplicates<TList, TReturn>(IEnumerable<TList> list, string propertyName)

    {

        return (IEnumerable<TReturn>)list

            .GroupBy(x => x.GetType().GetProperty(propertyName).GetValue(x))

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

            .Select(x => (TReturn)x.Key);

    }

}

​

public class MyClass

{

    public string Text {get;set;}

    public bool Flag {get;set;}

    public int Number {get;set;}

}